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Eocene to Miocene terrigenous inputs and export production: geochemical evidence from ODP Leg 177, Site 1090
Palaeogeography, Palaeoclimatology, Palaeoecology 182 (2002) 151^164 www.elsevier.com/locate/palaeo
Eocene to Miocene terrigenous inputs and export production: geochemical evidence from ODP Leg 177, Site 1090 Jennifer C. Latimer a;b; , Gabriel M. Filippelli b b
a Department of Geological Sciences, Indiana University, Bloomington, IN 47405, USA Department of Geology and Center for Earth and Environmental Sciences, Indiana University-Purdue University at Indianapolis, 723 W. Michigan St., Indianapolis, IN 46202, USA
Received 18 August 2000; accepted 5 December 2001
Abstract Changes in ocean circulation and climate during the Cenozoic led to the development of the Antarctic Circumpolar Current (ACC) and permanent Antarctic ice sheets. However, the timing of the opening of the Drake Passage and the establishment of the ACC is poorly constrained. We present geochemical proxies of terrigenous inputs and export production at a single site in the southeastern Atlantic Ocean that suggest the Drake Passage had opened to intermediate and deep water by 32.8 Ma. Two separate styles of sedimentation and geochemical composition of terrigenous material are observed, with the change in sediment geochemical characteristics occurring at 32.8 Ma. Middle to late Eocene records are highly variable, while latest Eocene to early Miocene records exhibit cyclic variations in elemental records and terrigenous material. Based on Al/Ti ratios, metal sources change from continental crust to oceanic crust sources at the Eocene/Oligocene transition, which we suggest reflects a change in deep-water circulation. Phosphorus/metal ratios indicate that there are two distinct intervals of enhanced export production, one in the middle Eocene and one throughout the Eocene/Oligocene transition. Elevated Al/Ti ratios, greater than any lithic source, in the middle Eocene provide evidence of particulate scavenging and thus increased export production. Barium ratios further support changes in productivity in the middle Eocene and at the Eocene/Oligocene transition. Permanent changes in the Ba concentration record in the early Oligocene further support a change in deep-water circulation due to the opening and deepening of the Drake Passage at 32.8 Ma. : 2002 Elsevier Science B.V. All rights reserved. Keywords: geochemistry; Drake Passage; Atlantic Ocean; phosphorus
1. Introduction Understanding Eocene to Miocene variations in * Corresponding author. present address: Indiana/Purdue University at Indianapolis, Indianapolis, Indiana, USA, phone: 317-274-7484, Fax: 317-274-7966. E-mail addresses: [email protected] (J.C. Latimer), g¢[email protected] (G.M. Filippelli).
the Southern Ocean is important because paleoceanographic and tectonic changes on these time scales resulted in the formation of the Antarctic Circum-polar Current (ACC) and the thermal isolation of the Antarctic continent. Due to the reorganization of the continents, the Tasmanian Seaway and the Drake Passage opened to deep water during the Cenozoic eventually leading to the establishment of circum-Antarctic currents,
0031-0182 / 02 / $ ^ see front matter : 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 1 - 0 1 8 2 ( 0 1 ) 0 0 4 9 3 - X
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which prevented tropical heat from reaching Antarctica and led to the development of Antarctic ice sheets (Kennett and Shackleton, 1976; Lawver et al., 1992). The Eocene/Oligocene boundary marks the onset of extensive East Antarctic glaciation, evidenced by oxygen isotope excursions and ice-rafted debris, and the transition from the greenhouse world to the icehouse world (Kennett and Shackleton, 1976). Cooling progressed through the Oligocene with several major cooling events occurring during the Eocene/Oligocene transition, the middle Oligocene, and the Oligocene/Miocene transition (Miller et al., 1991; Wright and Miller, 1993). Temperatures appeared to have warmed in the early Miocene, with a return to glacial conditions in the middle Miocene (Zachos et al., 1994). Still poorly known, however, is the relationship and feedbacks between ocean processes (i.e. circulation, productivity) and terrestrial processes (i.e. continental weathering, carbon dynamics) during this Cenozoic transition from greenhouse to icehouse worlds (e.g. Raymo and Ruddiman, 1992; Filippelli and Delaney, 1994; Zachos et al., 1999). Before the ACC could fully develop, both the Tasmanian Seaway and the Drake Passage must have opened to deep water. The opening of the Tasmanian Seaway has been fairly well constrained by recent drilling in the area, with estimates of opening near the Eocene/Oligocene transition (Shipboard Scienti¢c Party, 2000). However, the opening of the Drake Passage is more equivocal, with estimates ranging from the late Eocene (Lawver et al., 1992) to near the Oligocene/Miocene transition (Barker and Burrell, 1977). The opening of the Drake Passage was apparently very complex and included a series of false starts and shallow connections between the south Atlantic and south Paci¢c oceans before the ¢nal permanent opening of the passage (Lawver et al., 1992). Using the accumulation of biogenic opal and benthic organisms, several studies in the Southern Ocean have investigated changes in productivity during the Eocene and early Oligocene (Barrera and Huber, 1993; Lazarus and Caulet, 1993; Diester-Haass et al., 1993; Diester-Haass, 1995; Diester-Haass and Zahn, 1996; Salamy and Za-
chos, 1999). Cyclic changes in productivity, on the order of 400^450-kyr periodicity, have been observed (Diester-Haass et al., 1993; Diester-Haass, 1995; Diester-Haass and Zahn, 1996). Isotope records have revealed a positive carbon isotopic shift near the Eocene/Oligocene boundary, which Salamy and Zachos (1999) attribute to an increase in marine productivity. They further suggest that increased export production resulted from increased terrigenous inputs supplying limiting micro-nutrients. Geochemical modeling experiments indicate that chemical weathering £uxes globally were somewhat lower during the Eocene than at present (Sloan et al., 1997); despite higher Eocene runo¡ values, the distribution of weathering-susceptible lithologies and concentrated modern runo¡ increased modern chemical weathering. Unfortunately, little is known about terrigenous inputs and continental weathering during this time, highlighting the importance of obtaining terrigenous sedimentation and productivity records in this greenhouse to icehouse transition. To ¢ll in some of the gaps in our understanding of terrigenous inputs and paleoproductivity, we investigated changes in P burial and terrigenous inputs from middle Eocene to the early Miocene, spanning both the Eocene/Oligocene and Oligocene/Miocene transitions at a single site in the Subantarctic southeastern Atlantic Ocean (Site 1090, ODP Leg 177) using bulk sediment geochemistry. Our objective was to examine sedimentologic and geochemical variations to help constrain the opening of the Drake Passage. A further goal was to examine the eventual development of the ACC and its role on sediment characteristics and potential export production signals. Site 1090 is ideally suited for this study, due to shallow burial, continuous sediment core recovery, and excellent age control for a Paleogene sequence (Gersonde et al., 1999).
2. Study site Site 1090 is located in the Subantarctic southeastern Atlantic Ocean on the southern £ank of the Agulhas Ridge (42‡54PS, 8‡53PE), at a present water depth of 3702 m (Fig. 1). The record is long
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Fig. 1. Map of the southeastern Atlantic Ocean, showing the location of Site 1090 (6‡2PS, 8‡E). The locations of Site 744 (61‡S, 82‡E) on Kerguelen Plateau and Site 689 (64‡S, 3‡E) on Maud Rise are also shown. The approximate positions of the presentday Subantarctic Front and the Antarctic Polar Front are shown.
and nearly continuous from the middle Eocene (recovered at maximum coring depth of 407 m composite depth (mcd)) to the early Miocene, which is truncated by a hiatus from the early^ middle Miocene to the late Pliocene at about 70 mcd. The middle Eocene (V44 Ma) to middle Miocene (V16 Ma) sediment sequence is subdivided into two distinct lithostratigraphic units. The lower unit, from 407 mcd (V44 Ma) to 350 mcd (V40 Ma), is a mud-bearing nannofossil ooze, with chalk and several chert layers (although this unit contains very little opal). Clay minerals are a signi¢cant component of the terrigenous material in this unit and exhibit a decrease from the base of this unit to the top (Shipboard Scienti¢c Party, 1999). Sedimentation rates were approximately 10 m Myr31 in this unit. The upper unit, from about 350 mcd (V40 Ma) to 70 mcd (V16 Ma), is a mud-bearing diatom and nannofossil ooze. The unit contains several distinct intervals of diatom ooze in the Eocene/Oligocene and early Miocene sediments. Sedimentation rates were typically about 10 m Myr31 in this unit, although more rapid sedimentation of late Eocene opal-rich sediments increased the rate to about 30 m Myr31 from 360 to 230 mcd. This site is above the calcium carbonate compensation depth (Gersonde et al., 1999), and may have been near or above the CCD throughout its history (Berger, 1977); thus carbonate sediment
dissolution will not signi¢cantly a¡ect the sediment records. It should be noted, however, that foraminifer abundance is low and preservation is poor in the Eocene to Oligocene section. Further, calcium carbonate variability seen particularly in the Eocene and the Eocene/Oligocene boundary sequences indicates brief intervals of extreme dissolution. The dearth of foraminifera in this interval also precludes high-resolution reconstructions of surface or deep isotopic geochemistry at this site.
3. Age model For the late Oligocene to early Miocene (71.59^ 209.52 mcd) section the age model of Billups et al. (in press), which is based on benthic foraminiferal oxygen isotope stratigraphy and magnetic polarity stratigraphy, was used. For the Eocene to early Oligocene section (220.49^409.27 mcd), calcareous nannofossil datums (Marino and Flores, in press) were used as control points. Linear sedimentation rates were assumed between control points and ages were assigned. At this time, the exact position of the Eocene/Oligocene boundary is not well constrained. Di¡erent biostratigraphic datums suggest di¡erent depths of the boundary, ranging from V258 mcd based on calcareous nannofossil stratigraphy (Marino and Flores, in press) to
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V270 mcd based on planktic foraminifera stratigraphy (Galeotti et al., in press). While we adopt the Marino and Flores datums for our age model, we admit that the Eocene/Oligocene boundary is actually within a 15-m section between 255 and 270 mcd. Our placement of the Eocene/Oligocene boundary potentially is in error by as much as 15 m and several hundred thousand years; however, the dissolution event in the earliest Oligocene from 250 to 254 mcd can be correlated with similar early Oligocene dissolution events at other sites in the Southern Ocean, for example, at sites located on Kerguelen Plateau and Maud Rise (Salamy and Zachos, 1999; Diester-Haass 1995, 1996; Ehrmann and Mackensen, 1992) further supporting our use of the Marino and Flores age model.
4. Methods Approximately 650 samples spanning both the
Eocene/Oligocene boundary and the Oligocene/ Miocene boundary were analyzed using a total sediment digestion and subsequent analysis by inductively coupled plasma^atomic emission spectrometry (ICP^AES). Elements analyzed for total concentrations include P, Fe, Al, Ti, Ba, Mn, Zn, Sr, Mg and Ca. Fifteen percent of the sediment digestions were analyzed as randomly chosen replicates. All total digestion replicates agreed within 5%. Elemental ratios were calculated and the average precision of the ratios was 3.6^5.5%. Samples were dried at 105‡C overnight and crushed. Approximately 0.1 g of each sample was dissolved using a CEM Corporation MDS 2000 Microwave Digestion System and concentrated trace metal grade HNO3 , HF, and HCl following EPA SW846 Method 3051. Samples were digested in Te£on reaction vessels using a combination of heat, pressure, and strong acids to dissolve the sediment. Once the digestion was complete, boric acid was added to stabilize the solutions. The samples were transferred to new
Fig. 2. Carbonate content of 1090 sediments based on Ca concentrations, the solid circles represent age control points based on oxygen isotope and magnetostratigraphy (Billups et al., submitted), the solid triangles represent age control points based on nannofossil datums (Marino and Flores, in press). The Eocene/Oligocene transition is shaded in gray. Several intervals of extremely low carbonate content are observed during the Oligocene; however, the dissolution event from 250 to 254 mcd occurs in the earliest Oligocene and can be correlated to dissolution events observed at other sites in the Southern Ocean. Galeotti et al. (in press) place the Eocene/Oligocene boundary at 270 mcd based on planktic foraminifera. We adopt the nannofossil-based biostratigraphy of Marino and Flores (in press) for this study.
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50-ml polypropylene centrifuge tubes and diluted to 50 ml with Milli-Q water. A Leeman Labs PS950 ICP^AES with a CETAC Corporation AT5000þ ultrasonic nebulizer was used to determine total elemental concentrations.
5. Results 5.1. Carbonate content Assuming carbonate is responsible for all Ca present, the weight percent of carbonate was calculated (Fig. 2). The calculated carbonate content is comparable to values obtained by standard coulometric techniques for carbonate content (Shipboard Scienti¢c Party, 1999). Using Ca concentrations as a proxy for carbonate appears not only to be valid, but also in this case yields a higher resolution record of carbonate content than is otherwise available. The preliminary Eocene/Oligocene boundary based on biostratigraphic reconstructions was placed at V240 mcd (Shipboard Scienti¢c Party, 1999). Our carbonate
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record reveals a decrease in carbonate content in the interval from 250 to 254 mcd, with values dropping to near zero compared to about 10% carbonate before and after this interval. Based on these low carbonate values and the lack of preserved foraminifera, this interval likely represents a signi¢cant dissolution event. Similar dissolution events are observed at other sites throughout the Southern Ocean just after the Eocene/Oligocene boundary (Salamy and Zachos, 1999; Diester-Haass, 1996). Because of the proximity of this dissolution event to the biostratigraphic datums of Marino and Flores (in press) the dissolution event we observe in the interval from 250 to 254 mcd likely occurs at the beginning of the Oligocene. There are several other intervals of very low carbonate content likely re£ecting carbonate dissolution during the Oligocene. 5.2. Terrigenous content Using the Al (3113 Wmol g31 ) and Ti (110.7 Wmol g31 ) concentrations found in average crust (Taylor and McLennan, 1985), we normalized the
Fig. 3. Seven point smoothed percent terrigenous determined by Al normalization plotted against age. Gray shaded area represents the Eocene/Oligocene transition. The record is more variable during the Eocene and becomes more cyclic in the Oligocene and Miocene. Values exceeding 100% are artifact of the normalization process, meaning the end-member chosen for normalization purposes does not necessarily represent the actual end-member source.
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Al and Ti concentrations to calculate percent terrigenous material. Both calculations produce the same downcore trends; however, the calculated values are di¡erent. Many of the Ti-normalized values exceed 100%, whereas the Al values are typically lower. Al-normalized values are presented in Fig. 3. The hazard of normalizing geochemical values, of course, is that the average end-member source does not always represent the actual end-member source, which explains the calculated values greater than 100%. Values we calculated, however, vary in parallel with those interpreted from magnetic susceptibility and color re£ectance proxies, and also correspond to lower resolution visual shipboard assessments of percent terrigenous material. The Al/Ti ratio can be used to identify source types of detrital material, as di¡erent rock types have di¡erent Al/Ti ratios (e.g. Taylor and McLennan, 1985). The Al/Ti record is marked by two dominant modes: relatively higher and more variable values from 44 to 32.8 Ma, and lower and generally constant values from 32.8 to
16 Ma (Fig. 4). The Al/Ti ratio during the later constant interval averages 6.5 P 4.0, similar to a basaltic or oceanic crust value (V7.3; Taylor and McLennan, 1985). The Al/Ti ratio during the earlier interval averages 13.4, similar to the average continental crust value of 15.8, but has a high degree of variability ( P 19.0) re£ective of several extremely high values (Fig. 4). The highest rock end-member value for Al/Ti ratios is about 40 from granites (Taylor and McLennan, 1985), leading Murray and Leinen (1996) and Murray et al. (1993) to suggest that extremely high values found in equatorial Paci¢c sediments are the result of Al scavenging during intervals of high export productivity. Al/Ti ratios less than 40 may provide information about detrital sources, in particular whether sources are continental or oceanic, while Al/Ti ratios greater than 40 may provide information about export production. Excluding the extremely high values in the earlier interval, the Al/Ti ratio is generally higher and more variable, and undergoes a permanent shift at about 32.8 Ma.
Fig. 4. Seven point smoothed Al/Ti plotted versus age. The Al/Ti ratio is highly variable during the middle Eocene with maximum values higher than lithogenic sources. Granites have the highest Al/Ti ratio of greater than 40, which is exceeded during the middle Eocene. Late Eocene Al/Ti ratios are similar to those found in average continental crust (V15.8) or average upper crust (V27) suggesting a continental source (Taylor and McLennan, 1985). Values from about the Eocene/Oligocene boundary to 16 Ma are generally lower (Al/Ti = 10) and more similar to oceanic sources, such as oceanic crust or basalt (Taylor and McLennan, 1985).
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Fig. 5. Scatter plots of P and Ba concentrations versus Ti concentrations. A positive correlation with Ti suggests P and Ba are at least partially associated with terrigenous material.
and Delaney, 1994; Fo«llmi, 1995; Filippelli, 1997). At Site 1090, P is correlated with Ti, indicating that much of the P appears to be deposited in conjunction with terrigenous material (Fig. 5). This is di¡erent from other open ocean settings like the equatorial Paci¢c, where sedimentary P is linked quite strongly to biological processes (Filippelli and Delaney, 1996), but agrees with results from several other Southern Ocean sites (Latimer and Filippelli, 1997). A better proxy of non-terrigenous (i.e. biological) P is perhaps the P/Al and P/Ti ratios, which scale P as a function of terrigenous content. Records of P and P/Al and P/Ti ratios reveal some marked di¡erences in these proxies. For example, the long-term P record reveals an increase during the middle Eocene, a decline to just after the Eocene/Oligocene boundary, and apparent cyclic variations during the Oligocene and Miocene (Fig. 6, top). In contrast, the P/Al record reveals high values in the middle Eocene, with a peak at about 42 Ma corresponding to a peak in the bulk P record, then low and relatively constant values from the middle Eocene to the middle Miocene broken only by signi¢cant peaks from about 34.5 to 33 Ma (Fig. 6, middle). The P/Ti ratio also reveals high values in the earliest part of the record, followed by some peaks during the late Eocene/early Oligocene and a gradual increase through the Miocene (Fig. 6, bottom). In terms of a paleoproductivity proxy, we favor the P/Al record because (1) the variation in the P concentration record is likely also responding to variations in terrigenous matter input and is thus an imperfect proxy of paleoproductivity, and (2) the Ti record does not provide the proper normalization (as discussed earlier) and may have a less well constrained source than Al.
5.3. P records
5.4. Ba records
Phosphorus (P) is a limiting nutrient for productivity (Holland, 1978; Broecker, 1982; Smith, 1984), and is likely the most important on geologic time scales (Tyrell, 1999). For this reason, sedimentary records of P and P geochemistry have been used as a measure of paleoproductivity (e.g. Moody et al., 1988; Ingall et al., 1993; Filippelli
The accumulation of barite in sediments appears to be related to organic C export in surface waters; however, the exact relationship is not known (Dymond et al., 1992). Barium concentrations are a proxy for barite and have been considered as a productivity proxy (e.g. Lea and Boyle, 1990; Paytan et al., 1996; McManus et al.,
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Fig. 6. Seven point smoothed P concentrations (top), P/Al ratios (middle) and P/Ti ratios (bottom) plotted versus age. Because P is positively correlated with Ti, a terrigenously derived metal, we use P/Al and P/Ti ratios as proxies for export production rather than P concentrations. Most concentrations range from 20 to 65 Wmol P g31 , with elevated concentrations in the late early Miocene. Elevated P/Al and P/Ti ratios are interpreted as increases in export production. The most notable increases occur during the middle Eocene (V44^42 Ma) and across the Eocene/Oligocene transition (shaded area).
1998, 1999). Barium has a nutrient-type water column pro¢le with concentrations typically peaking below 2000 m. Preservation of Ba in sediments is dependent on several factors including bottom water oxygen and pore water sulfate concentrations, requiring the careful application and interpretation of Ba records as a productivity proxy (McManus et al., 1998). When pore water sulfate becomes depleted, barite is highly susceptible to dissolution ; however, downcore pore water sulfate pro¢les never go to zero at Site 1090 (Shipboard Scienti¢c Party, 1999). In younger, glacial/ interglacial sediments, Nu«rnberg et al. (1997) il-
lustrated that Ba is a valid productivity proxy in the south Atlantic sector of the Southern Ocean. Our Ba concentration record exhibits a sharp and permanent change at 33 Ma, from low concentrations (averaging 5 Wmol g31 ) with low variability before 33 Ma to high concentrations (averaging 21 Wmol g31 ) and relatively high variability after 33 Ma (Fig. 7, top). Similar to P, Ba concentrations are positively correlated with terrigenous material (r2 = 0.54). For this reason, Ba concentrations were normalized to terrigenous material by calculating Ba/Al and Ba/Ti ratios (Schroeder et al., 1997). Despite
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Fig. 7. Seven point smoothed Ba concentrations (top) Ba/Al ratios (middle) and Ba/Ti ratios (bottom) plotted versus age. Ba concentrations undergo a dramatic and permanent change at 32.8 Ma. Prior to 32.8 Ma, Ba concentrations are low, averaging 5 Wmol g31 , and after 32.8 Ma Ba concentrations are higher and more variable, averaging 21 Wmol g31 . Ba/Al and Ba/Ti ratios exhibit similar peaks to those seen in P/Al and P/Ti records (Fig. 6), with maxima from V44 to 42 Ma and across the Eocene/ Oligocene transition, which we interpret as increases in export production.
di¡erences between centrations, the P similar trends, with Eocene and across tion.
the total P and total Ba conratios and Ba ratios exhibit high values during the middle the Eocene/Oligocene transi-
6. Discussion 6.1. Variations in terrigenous/detrital material Increases in terrigenous supply have been suggested as a mechanism providing limiting trace nutrients to the surface ocean (Martin, 1990). A shift in the style of terrigenous sedimentation and
the geochemical composition of the terrigenous material occurs at Site 1090. Note that by ‘terrigenous’, we mean any input from a rock source, including terrestrial input, turbiditic input, and even hydrothermal and basaltic input. Clay content decreases during the middle to late Eocene, observed in visual and color re£ectance records as a decrease in sediment ‘redness’ but also determined by shipboard X-ray di¡raction analyses (Shipboard Scienti¢c Party, 1999). This decrease in clay content occurs despite a slight increase in terrigenous content. Thus, the relative content of the non-clay mineral component increases in this interval, possibly indicating a trend toward an increase in physically weathered mate-
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rial reaching this site and a decrease in the chemically weathered material. A similar change has been observed from sites much nearer to Antarctica, although the transition from chemical weathering-dominated to physically weathering-dominated terrigenous input occurs much later, from about 36 to 33 Ma (Ehrmann and Mackensen, 1992), than we observe here (from about 44 to 40 Ma). Beginning in the latest Eocene, terrigenous content becomes more cyclic compared to the highly variable terrigenous records prior to the Eocene/ Oligocene transition. In combination with the change in terrigenous content, a shift in the Al/ Ti ratio is observed. Excluding the very high Al/Ti ratios greater that 40, we still see a shift from continental sources prior to the Eocene/Oligocene transition to lower Al/Ti ratios with a likely oceanic source after the transition. This shift can only be interpreted as a source change from more continental crust to more oceanic crust detrital material at this site. The likely force driving this source change is deep-water circulation. An intriguing hypothesis is that 33 Ma marks the sudden onset of signi¢cant Paci¢c water bathing Site 1090. A change in deep current £ow such as this would bring entrained terrigenous material from the west, and this material was enriched in basaltic components (a likely scenario given rifting and ridge-building during the opening of the Drake Passage). We suggest, therefore, that signi¢cant ACC production began at 33 Ma, and remained strong and persistent from 33 to at least 16 Ma. 6.2. Export production variations A variety of proxies are used to assess productivity, including sedimentological (e.g. biogenic accumulation rates), biological (e.g. microfossil assemblages), and geochemical (e.g. carbon and nitrogen isotopes, biogenically enhanced trace metals, phosphorus) measures. Our approach here will focus on phosphorus, barium, and trace metal geochemistry, in the hope that these proxies can be integrated with other proxies at this site to develop a coherent picture of productivity variations through time.
Based on the P/Al and Ba/Al record, we identify two intervals of enhanced export production. The ¢rst interval occurs in the middle Eocene, from the base of our record at 44 Ma to approximately 42 Ma (Figs. 6 and 7). Sedimentologically, this interval is characterized by muddy nannofossil ooze with some chert, and contains a relatively high clay mineral content compared to the rest of the record. The P/Al ratios peak to above 1.0 (their highest values of the entire record), signi¢cantly above their average value of 0.1. The crustal value for P/Ti ratio is 0.01, and that found in carbonate rocks is 0.07 (Faure, 1999); the extremely high middle Eocene values thus indicate excess P burial and hence enhanced export production. This is further supported by elevated Ba/ Al and Ba/Ti ratios during these same intervals. This assertion is reinforced by the Al/Ti values during this interval, which peak to values well above any source other than is found during particulate scavenging and high export production (Murray et al., 1995). In fact, the P/Al ratios are small compared to what they could be, considering that the Al/Ti values indicate excess Al deposition that would normally drive down the P/Al ratio. High export production in the Southern Ocean during the middle Eocene is not without precedence: Diester-Haass and Zahn (1996) found peaks in paleoproductivity from about 44 to 41.6 Ma at the Maud Rise Site 689. A second high export production interval occurs from about 34.5 to 32.8 Ma (Fig. 6, middle and bottom). This event occurs as several discrete peaks in P/Al and P/Ti occurring before and after the Eocene/Oligocene boundary, coincident with peaks in P concentration (Fig. 6, top). Ba/Al and Ba/Ti ratios reveal similar discrete peaks across the Eocene/Oligocene transition further supporting short intervals of enhanced export production. Unlike the sharp peak in productivity proxies observed just after the Eocene/Oligocene boundary by Salamy and Zachos (1999), our results correspond more to the paleoproductivity record of Diester-Haass and Zahn (1996) from the Maud Rise, with similar sharp peaks beginning in the late Eocene. Based on these Subantarctic and Antarctic records, therefore, it appears that export production was indeed tem-
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porarily enhanced around the Eocene/Oligocene boundary, coincident with changes in oxygen isotopic composition re£ecting cooling and the onset of extensive Antarctic glaciation (e.g. Stott et al., 1992; Zachos et al., 1996). This enhancement, however, appears as several short (i.e. V100 000^200 000 yr) productivity peaks with a return to previous lower productivity conditions. Diester-Haass and Zahn (1996) attribute these transient increases in productivity to the ice build-up phenomenon, either strong oceanic overturn or the sudden input of dissolved nutrients during initial glaciation. Salamy and Zachos (1999) interpret the sharp productivity pulse just after the Eocene/Oligocene boundary to enhanced input of eolian material and possibly Fe-fertilization. Iron concentrations do increase during this interval (Fig. 8) coincident with P increases, but we know too little about the source of Fe and the relationship between Fe and P to brandish this as the smoking gun for the observed productivity increases.
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6.3. Ocean circulation variations The development of the ACC is clearly related to the opening of the Drake Passage, but much controversy exists about the style and timing of Drake Passage opening. For example, how wide and deep must the Drake Passage have been to allow adequate water £ux for a circum-polar current to develop and self-stabilize? Further, when did this occur? Several studies place the opening of the Drake Passage at about 30 Ma (Barker and Burrell, 1977; Lawver et al., 1992), while others note environmental and geochemical data indicating an earlier opening of about 37 Ma (e.g. Diester-Haass and Zahn, 1996). Still others suggest the opening of the Drake Passage occurred much later during the Oligocene near the Oligocene/Miocene boundary (Barker and Burrell, 1977). This is a topic of ongoing debate, as it is critical to understanding the thermal isolation of Antarctica and the global climatic response to that thermal isolation. For this reason, we wade into this
Fig. 8. Fe (solid triangles) and P (open circles) concentrations plotted against age. Salamy and Zachos (1999) suggested eolian Fe-fertilization increased productivity in the earliest Oligocene. From 34 to 32.8 Ma, Fe and P concentrations appear positively correlated, with peaks in both concentrations coincident; however, the exact relationship between Fe and productivity at this time is unknown.
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controversy with several observations from Site 1090. First, as mentioned before, the terrigenous records indicate a permanent shift in geochemical characteristics of the sediments at Site 1090 occurring at 32.8 Ma. We suggest that this marks the opening of the Drake Passage and the eastward transport of basaltic terrigenous material from the rifting regions to the west of Site 1090. Ba concentration records exhibit a permanent and dramatic shift at 32.8 Ma. We suggest that the Drake Passage was open to intermediate and deep waters at 32.8 Ma. We base this interpretation on the Ba records from this site. We suggest that this transition represents an adequate Drake Passage opening to allow for circulation of intermediate and deep waters with relatively higher preformed nutrient contents through the passage. This interpretation can be tested further by examining the same type of proxies in other Subantarctic, Subtropical, and Antarctic sites. One unknown, however, in this 32.8-Ma interpretation is that Ba is correlated to some degree with terrigenous matter (r2 = 0.54), and thus some of the Ba signal is simply a terrigenous signal (which we use to denote a 32.8-Ma opening of the Drake Passage). But the Ba/Ti record does reveal a unique character to the record before and after 32.8 Ma (Fig. 7, bottom), though not as clearly as the Ba record itself. Furthermore, the Ba/Al and Ba/Ti records re£ect several of the productivity maxima observed from other proxies in the middle Eocene and in the late Eocene/early Oligocene, further indicating its partial nutrient connection at this site.
7. Conclusions Geochemical proxies were used to construct high-resolution records of terrigenous content and export production at ODP Site 1090 for the middle Eocene through the early Miocene. Several interpretations arise from these results: (1) Terrigenous records indicate di¡erent styles and compositions of terrigenous material before and after 32.8 Ma. This change in sediment characteristics is supported by clay mineral content,
which suggests a change in dominance of chemically weathered to physically weathered material reaching the site. Al/Ti ratios prior to 32.8 Ma are more variable, relatively higher, and more similar to continental crust values, while Al/Ti ratios after 32.8 Ma remain constant and near ocean crust values. (2) Paleoproductivity proxies suggest two intervals of enhanced export production. Elevated P/Al ratios identify intervals of enhanced export production occurring from 44 to 42 Ma and from 34.5 to 32.8 Ma. Elevated Al/Ti ratios, perhaps indicative of particulate scavenging in the middle Eocene, further support increased export production at this time. Across the Eocene/Oligocene transition, export production appears to increase based on P and Ba ratios and concentrations. Both intervals of enhanced productivity correspond to records from other sites in the Southern Ocean. (3) The di¡erences in the style and characteristics of the geochemical records at 32.8 Ma suggest changes in deep-water circulation. Records of concentration and terrigenous content reveal a change from highly variable to more cyclic records beginning at the Eocene/Oligocene transition. The change in average Al/Ti ratios indicates a change in detrital source, from continental to oceanic crustal sources, that we suggest resulted from a change in deep-water circulation. Finally, Ba concentration records reveal a dramatic shift from low values to values that are highly variable and on average four times greater in magnitude than prior to 32.8 Ma. We suggest the Eocene/ Oligocene transition appears to mark the initiation of the ACC with the Drake Passage open to intermediate and deep waters by 32.8 Ma.
Acknowledgements We acknowledge research support from JOI, Inc. (Schlanger Ocean Drilling Fellowship to J.C.L.), JOI/USSSP (to G.M.F.), NSF (OCE 9711957 to G.M.F.) and the donors of the American Chemical Society through the Petroleum Research Fund. Comments from A. Bellanca, R. Gersonde, D. Hodell, R. Murray, C.
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Robert, J. Wright and an anonymous reviewer greatly improved this manuscript. Also thanks to Robyn Atkins, Rosalice Buehrer, and Sara Slater for their assistance. References Barker, P.F., Burrell, J., 1977. The opening of the Drake Passage. Mar. Geol. 25, 15^34. Barrera, E., Huber, B.T., 1993. Eocene to Oligocene oceanography and temperatures in the Antarctic Indian Ocean. In: Kennett, J.P., Warnke, D.A. (Eds.), The Antarctic Paleoenvironment: A Perspective on Global Change. Antarct. Res. Ser. 60, 49^65. Berger, W.H., 1977. Carbon dioxide excursions and the deepsea record: aspects of the problem. In: Anderson, N.R., Malaho¡, A. (Eds.), Fate of Fossil Fuel CO2 . Plenum, NY, pp. 505^542. Billups, K., Channell, J., Zachos, J., in press. Late Oligocene to Early Miocene Paleoceanography from the Subantarctic South Atlantic. Paleoceanography. Broecker, W.S., 1982. Ocean chemistry during glacial time. Geochim. Cosmochim. Acta 46, 1689^1705. Diester-Haass, L., 1995. Middle Eocene to early Oligocene paleoceanography of the Antarctic Ocean (Maud Rise, ODP Leg 113, Site 689): change from a low to a high productivity ocean. Palaeogeogr. Palaeoclimatol. Palaeoecol. 113, 311^ 334. Diester-Haass, L., 1996. Late-Eocene^Oligocene paleoceanography in the southern Indian Ocean (ODP Site 744). Mar. Geol. 130, 99^119. Diester-Haass, L., Robert, C., Chamley, H., 1993. Paleoceanographic and paleoclimatic evolution in the Weddell Sea (Antarctica) during the middle Eocene^late Oligocene, from a coarse sediment fraction and clay mineral data (ODP Site 689). Mar. Geol. 114, 233^250. Diester-Haass, L., Zahn, R., 1996. Eocene^Oligocene transition in the Southern Ocean: history of water mass circulation and biological productivity. Geology 24, 163^166. Dymond, J., Suess, E., Lyle, M., 1992. Barium in deep-sea sediment: A geochemical proxy for paleoproductivity. Paleoceanography 7, 163^181. Ehrmann, W.U., Mackensen, A., 1992. Sedimentologic evidence for the formation of an East Antarctic ice sheet in Eocene/Oligocene time. Palaeogeogr. Palaeoclimatol. Palaeoecol. 93, 85^112. Faure, G., 1999. Principles and Applications of Geochemistry. Prentice Hall, Upper Saddle River, NJ. Filippelli, G.M., 1997. Intensi¢cation of the Asian Monsoon and a chemical weathering event in the late Miocene/early Pliocene: Implications for late Neogene climate change. Geology 25, 27^30. Filippelli, G.M., Delaney, M.L., 1994. The oceanic phosphorus cycle and continental weathering during the Neogene. Paleoceanography 9, 643^652.
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Filippelli, G.M., Delaney, M.L., 1996. Phosphorus geochemistry of equatorial Paci¢c sediments. Geochim. Cosmochim. Acta 60, 1479^1495. Fo«llmi, K.B., 1995. 160 m.y. record of marine sedimentary phosphorus burial Coupling of climate and continental weathering under greenhouse and icehouse conditions. Geology 23, 859^862. Galeotti, S., Cocciono, R., Gersonde, R., in press. Middle Eocene^early Pliocene planktic foraminiferal biostratigraphy at ODP Leg 177 Site 1090, Agulhas Ridge. Mar. Micropaleontol. Gersonde, R., Hodell, D.A., Blum, P. et al., 1999. Proc. ODP, Init. Rep., 177. College Station, TX. Holland, H.D., 1978. The Chemical Evolution of the Atmosphere and Oceans. Princeton University Press, 582 pp. Ingall, E.D., Bustin, R.M., VanCappellan, P., 1993. In£uence of water column anoxia on the burial and preservation of carbon and phosphorus in marine shales. Geochim. Cosmochim. Acta 57, 303^316. Kennett, J.P., Shackleton, N.J., 1976. Oxygen isotopic evidence for the development of the psychrosphere 38 Myr ago. Science 260, 513^515. Latimer, J.C., Filippelli, G.M., 1997. Phosphorus and iron sedimentation in the Southern Ocean on glacial/interglacial time scales. EOS (Abstr.) 78, S191. Lawver, L.A., Gahagan, L.M., Co⁄n, M.F., 1992. The development of paleoseaways around Antarctica. In: Kennett, J.P., Warnke, D.A. (Eds.), The Antarctic Paleoenvironment: A Perspective on Global Change. Antarct. Res. Ser. 56, 7^30. Lazarus, D., Caulet, L.P., 1993. Cenozoic Southern Ocean reconstructions from sedimentologic, radiolarian, and other microfossil data. In: Kennett, J.P., Warnke, D.A. (Eds.), The Antarctic Paleoenvironment: A Perspective on Global Change. Antarct. Res. Ser. 60, 145^174. Lea, D.W., Boyle, E.A., 1990. Foraminiferal reconstructions of barium distributions in water masses of the glacial ocean. Paleoceanography 5, 719^742. Marino, M., Flores, J.A., in press. Middle Eocene to early Oligocene calcareous nannofossil stratigraphy at Leg 177 Site 1090. Mar. Micropaleontol. Martin, J.H., 1990. Glacial-interglacial CO2 change: the iron hypothesis. Paleoceanography 5, 1^13. McManus, J., Berelson, W.M., Hammond, D.E., Klinkhammer, G.P., 1999. Barium cycling in the North Paci¢c: Implications for the utility of Ba as a paleoproductivity and paleoalkalinity proxy. Paleoceanography 14, 53^61. McManus, J., Berelson, W.M., Klinkhammer, G.P., Johnson, K.J., Coale, K.H., Anderson, R.F., Kumar, N., Brumsack, H.J., McCorkle, D.C., Rushdi, A., 1998. Geochemistry of barium in marine sediments: Implications for its use as a paleoproxy. Geochim. Cosmochim. Acta 62, 3453^3473. Miller, K.G., Wright, J.D., Fairbanks, R.G., 1991. Unlocking the ice house: Oligocene-Miocene oxygen isotopes, eustasy, and margin erosion. J. Geophys. Res. 96, 6829^6848. Moody, J.B., Chaboudy, L.R., Worsley, T.R., 1988. Paci¢c pelagic phosphorus accumulation during the last 10 M.Y.. Paleoceanography 3, 113^136.
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Murray, R.W., Leinen, M., 1996. Scavenged excess Al and its relationship to bulk Ti in biogenic sediment from the central equatorial Paci¢c Ocean. Geochim. Cosmochim. Acta 60, 3869^3878. Murray, R.W., Leinen, M., Isern, A.R., 1993. Biogenic £ux of Al to sediment in the central equatorial Paci¢c Ocean: evidence for increased productivity during glacial periods. Paleoceanography 8, 651^670. Murray, R.W., Leinen, M., Murray, D.W., Mix, A.C., Knowlton, C.W., 1995. Terrigenous Fe input and biogenic sedimentation in the glacial and interglacial equatorial Paci¢c Ocean. Glob. Biogeochem. Cycles 9, 667^684. Nu«rnberg, C.C., Bohrman, G., Schlu«ter, M., Frank, M., 1997. Barium accumulation in the Atlantic sector of the Southern Ocean: Results from 190,000-year records. Paleoceanography 12, 594^603. Paytan, A., Kastner, M., Chavez, F.P., 1996. Glacial to interglacial £uctuations in productivity in the equatorial Paci¢c as indicated by marine barite. Science 274, 1355^1357. Raymo, M.E., Ruddiman, W.F., 1992. Tectonic forcing of late Cenozoic climate. Nature 359, 117^122. Salamy, K.A., Zachos, J.C., 1999. Latest Eocene^Early Oligocene climate change and Southern Ocean fertility: inferences from sediment accumulation and stable isotope data. Palaeogeogr. Palaeoclimatol. Palaeoecol. 145, 1^77. Schroeder, J.O., Murray, R.W., Leinen, M., P£aum, R.C., Janecek, T.R., 1997. Barium in equatorial Paci¢c carbonate sediment: Terrigenous, oxide, and biogenic associations. Paleoceanography 12, 125^146. Shipboard Scienti¢c Party, 1999. Site 1090. In: Gersonde, R., Hodell, D.A., Blum, P. et al., Proc. ODP, Init. Rep., 177. College Station, TX, pp. 1^111. Shipboard Scienti¢c Party, 2000. Leg 189 Preliminary Report: The Tasmanian Seaway between Australia and Antarctica ^
Paleoclimate and paleoceanography. ODP Prelim. Rep., 89 (online). Sloan, L.C., Bluth, G.J.S., Filippelli, G.M., 1997. A comparison of spatially resolved and global mean reconstructions of continental denudation under ice free and present conditions. Paleoceanography 12, 147^160. Smith, S.V., 1984. Phosphorus versus nitrogen limitation in the marine environment. Limnol. Oceanogr. 29, 1149^1160. Stott, L.D., Kennett, J.P., Shackleton, N.J., Cor¢eld, R.M., 1992. The evolution of Antarctic surface waters during the Paleogene; inferences from the stable isotopic composition of planktonic foraminifers, ODP Leg 113. In: Proc. ODP, Sci. Results, 113, 849^863. Taylor, S.R., McLennan, S.M., 1985. The Continental Crust: Its Composition and Evolution. Blackwell, Malden, Mass. Tyrell, T., 1999. The relative in£uence of nitrogen and phosphorus on oceanic primary production. Nature 400, 525^ 531. Wright, J.D., Miller, K.G., 1993. Southern Ocean in£uences on late Eocene to Miocene deepwater circulation. In: Kennet, J.P., Warnke, D.A. (Eds.), The Antarctic Paleoenvironment: A Perspective on Global Change. Antarct. Res. Ser. 60, 1^25. Zachos, J.C., Opdyke, B.N., Quinn, T.M., Jones, C.E., Halliday, A.N., 1999. Early Cenozoic glaciation, Antarctic weathering, and seawater 87 Sr/86 Sr: is there a link? Chem. Geol. 161, 165^180. Zachos, J.C., Quinn, T.M., Salamy, K.A., 1996. High-resolution (104 years) deep-sea foraminiferal stable isotope records of the Eocene^Oligocene climate transition. Paleoceanography 11, 251^266. Zachos, J.C., Stott, L.D., Lohmann, K.C., 1994. Evolution of early Cenozoic marine temperatures. Paleoceanography 9, 353^387.
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Eocene to Miocene terrigenous inputs and export production: geochemical evidence from ODP Leg 177, Site 1090 Jennifer C. Latimer a;b; , Gabriel M. Filippelli b b
a Department of Geological Sciences, Indiana University, Bloomington, IN 47405, USA Department of Geology and Center for Earth and Environmental Sciences, Indiana University-Purdue University at Indianapolis, 723 W. Michigan St., Indianapolis, IN 46202, USA
Received 18 August 2000; accepted 5 December 2001
Abstract Changes in ocean circulation and climate during the Cenozoic led to the development of the Antarctic Circumpolar Current (ACC) and permanent Antarctic ice sheets. However, the timing of the opening of the Drake Passage and the establishment of the ACC is poorly constrained. We present geochemical proxies of terrigenous inputs and export production at a single site in the southeastern Atlantic Ocean that suggest the Drake Passage had opened to intermediate and deep water by 32.8 Ma. Two separate styles of sedimentation and geochemical composition of terrigenous material are observed, with the change in sediment geochemical characteristics occurring at 32.8 Ma. Middle to late Eocene records are highly variable, while latest Eocene to early Miocene records exhibit cyclic variations in elemental records and terrigenous material. Based on Al/Ti ratios, metal sources change from continental crust to oceanic crust sources at the Eocene/Oligocene transition, which we suggest reflects a change in deep-water circulation. Phosphorus/metal ratios indicate that there are two distinct intervals of enhanced export production, one in the middle Eocene and one throughout the Eocene/Oligocene transition. Elevated Al/Ti ratios, greater than any lithic source, in the middle Eocene provide evidence of particulate scavenging and thus increased export production. Barium ratios further support changes in productivity in the middle Eocene and at the Eocene/Oligocene transition. Permanent changes in the Ba concentration record in the early Oligocene further support a change in deep-water circulation due to the opening and deepening of the Drake Passage at 32.8 Ma. : 2002 Elsevier Science B.V. All rights reserved. Keywords: geochemistry; Drake Passage; Atlantic Ocean; phosphorus
1. Introduction Understanding Eocene to Miocene variations in * Corresponding author. present address: Indiana/Purdue University at Indianapolis, Indianapolis, Indiana, USA, phone: 317-274-7484, Fax: 317-274-7966. E-mail addresses: [email protected] (J.C. Latimer), g¢[email protected] (G.M. Filippelli).
the Southern Ocean is important because paleoceanographic and tectonic changes on these time scales resulted in the formation of the Antarctic Circum-polar Current (ACC) and the thermal isolation of the Antarctic continent. Due to the reorganization of the continents, the Tasmanian Seaway and the Drake Passage opened to deep water during the Cenozoic eventually leading to the establishment of circum-Antarctic currents,
0031-0182 / 02 / $ ^ see front matter : 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 1 - 0 1 8 2 ( 0 1 ) 0 0 4 9 3 - X
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which prevented tropical heat from reaching Antarctica and led to the development of Antarctic ice sheets (Kennett and Shackleton, 1976; Lawver et al., 1992). The Eocene/Oligocene boundary marks the onset of extensive East Antarctic glaciation, evidenced by oxygen isotope excursions and ice-rafted debris, and the transition from the greenhouse world to the icehouse world (Kennett and Shackleton, 1976). Cooling progressed through the Oligocene with several major cooling events occurring during the Eocene/Oligocene transition, the middle Oligocene, and the Oligocene/Miocene transition (Miller et al., 1991; Wright and Miller, 1993). Temperatures appeared to have warmed in the early Miocene, with a return to glacial conditions in the middle Miocene (Zachos et al., 1994). Still poorly known, however, is the relationship and feedbacks between ocean processes (i.e. circulation, productivity) and terrestrial processes (i.e. continental weathering, carbon dynamics) during this Cenozoic transition from greenhouse to icehouse worlds (e.g. Raymo and Ruddiman, 1992; Filippelli and Delaney, 1994; Zachos et al., 1999). Before the ACC could fully develop, both the Tasmanian Seaway and the Drake Passage must have opened to deep water. The opening of the Tasmanian Seaway has been fairly well constrained by recent drilling in the area, with estimates of opening near the Eocene/Oligocene transition (Shipboard Scienti¢c Party, 2000). However, the opening of the Drake Passage is more equivocal, with estimates ranging from the late Eocene (Lawver et al., 1992) to near the Oligocene/Miocene transition (Barker and Burrell, 1977). The opening of the Drake Passage was apparently very complex and included a series of false starts and shallow connections between the south Atlantic and south Paci¢c oceans before the ¢nal permanent opening of the passage (Lawver et al., 1992). Using the accumulation of biogenic opal and benthic organisms, several studies in the Southern Ocean have investigated changes in productivity during the Eocene and early Oligocene (Barrera and Huber, 1993; Lazarus and Caulet, 1993; Diester-Haass et al., 1993; Diester-Haass, 1995; Diester-Haass and Zahn, 1996; Salamy and Za-
chos, 1999). Cyclic changes in productivity, on the order of 400^450-kyr periodicity, have been observed (Diester-Haass et al., 1993; Diester-Haass, 1995; Diester-Haass and Zahn, 1996). Isotope records have revealed a positive carbon isotopic shift near the Eocene/Oligocene boundary, which Salamy and Zachos (1999) attribute to an increase in marine productivity. They further suggest that increased export production resulted from increased terrigenous inputs supplying limiting micro-nutrients. Geochemical modeling experiments indicate that chemical weathering £uxes globally were somewhat lower during the Eocene than at present (Sloan et al., 1997); despite higher Eocene runo¡ values, the distribution of weathering-susceptible lithologies and concentrated modern runo¡ increased modern chemical weathering. Unfortunately, little is known about terrigenous inputs and continental weathering during this time, highlighting the importance of obtaining terrigenous sedimentation and productivity records in this greenhouse to icehouse transition. To ¢ll in some of the gaps in our understanding of terrigenous inputs and paleoproductivity, we investigated changes in P burial and terrigenous inputs from middle Eocene to the early Miocene, spanning both the Eocene/Oligocene and Oligocene/Miocene transitions at a single site in the Subantarctic southeastern Atlantic Ocean (Site 1090, ODP Leg 177) using bulk sediment geochemistry. Our objective was to examine sedimentologic and geochemical variations to help constrain the opening of the Drake Passage. A further goal was to examine the eventual development of the ACC and its role on sediment characteristics and potential export production signals. Site 1090 is ideally suited for this study, due to shallow burial, continuous sediment core recovery, and excellent age control for a Paleogene sequence (Gersonde et al., 1999).
2. Study site Site 1090 is located in the Subantarctic southeastern Atlantic Ocean on the southern £ank of the Agulhas Ridge (42‡54PS, 8‡53PE), at a present water depth of 3702 m (Fig. 1). The record is long
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Fig. 1. Map of the southeastern Atlantic Ocean, showing the location of Site 1090 (6‡2PS, 8‡E). The locations of Site 744 (61‡S, 82‡E) on Kerguelen Plateau and Site 689 (64‡S, 3‡E) on Maud Rise are also shown. The approximate positions of the presentday Subantarctic Front and the Antarctic Polar Front are shown.
and nearly continuous from the middle Eocene (recovered at maximum coring depth of 407 m composite depth (mcd)) to the early Miocene, which is truncated by a hiatus from the early^ middle Miocene to the late Pliocene at about 70 mcd. The middle Eocene (V44 Ma) to middle Miocene (V16 Ma) sediment sequence is subdivided into two distinct lithostratigraphic units. The lower unit, from 407 mcd (V44 Ma) to 350 mcd (V40 Ma), is a mud-bearing nannofossil ooze, with chalk and several chert layers (although this unit contains very little opal). Clay minerals are a signi¢cant component of the terrigenous material in this unit and exhibit a decrease from the base of this unit to the top (Shipboard Scienti¢c Party, 1999). Sedimentation rates were approximately 10 m Myr31 in this unit. The upper unit, from about 350 mcd (V40 Ma) to 70 mcd (V16 Ma), is a mud-bearing diatom and nannofossil ooze. The unit contains several distinct intervals of diatom ooze in the Eocene/Oligocene and early Miocene sediments. Sedimentation rates were typically about 10 m Myr31 in this unit, although more rapid sedimentation of late Eocene opal-rich sediments increased the rate to about 30 m Myr31 from 360 to 230 mcd. This site is above the calcium carbonate compensation depth (Gersonde et al., 1999), and may have been near or above the CCD throughout its history (Berger, 1977); thus carbonate sediment
dissolution will not signi¢cantly a¡ect the sediment records. It should be noted, however, that foraminifer abundance is low and preservation is poor in the Eocene to Oligocene section. Further, calcium carbonate variability seen particularly in the Eocene and the Eocene/Oligocene boundary sequences indicates brief intervals of extreme dissolution. The dearth of foraminifera in this interval also precludes high-resolution reconstructions of surface or deep isotopic geochemistry at this site.
3. Age model For the late Oligocene to early Miocene (71.59^ 209.52 mcd) section the age model of Billups et al. (in press), which is based on benthic foraminiferal oxygen isotope stratigraphy and magnetic polarity stratigraphy, was used. For the Eocene to early Oligocene section (220.49^409.27 mcd), calcareous nannofossil datums (Marino and Flores, in press) were used as control points. Linear sedimentation rates were assumed between control points and ages were assigned. At this time, the exact position of the Eocene/Oligocene boundary is not well constrained. Di¡erent biostratigraphic datums suggest di¡erent depths of the boundary, ranging from V258 mcd based on calcareous nannofossil stratigraphy (Marino and Flores, in press) to
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V270 mcd based on planktic foraminifera stratigraphy (Galeotti et al., in press). While we adopt the Marino and Flores datums for our age model, we admit that the Eocene/Oligocene boundary is actually within a 15-m section between 255 and 270 mcd. Our placement of the Eocene/Oligocene boundary potentially is in error by as much as 15 m and several hundred thousand years; however, the dissolution event in the earliest Oligocene from 250 to 254 mcd can be correlated with similar early Oligocene dissolution events at other sites in the Southern Ocean, for example, at sites located on Kerguelen Plateau and Maud Rise (Salamy and Zachos, 1999; Diester-Haass 1995, 1996; Ehrmann and Mackensen, 1992) further supporting our use of the Marino and Flores age model.
4. Methods Approximately 650 samples spanning both the
Eocene/Oligocene boundary and the Oligocene/ Miocene boundary were analyzed using a total sediment digestion and subsequent analysis by inductively coupled plasma^atomic emission spectrometry (ICP^AES). Elements analyzed for total concentrations include P, Fe, Al, Ti, Ba, Mn, Zn, Sr, Mg and Ca. Fifteen percent of the sediment digestions were analyzed as randomly chosen replicates. All total digestion replicates agreed within 5%. Elemental ratios were calculated and the average precision of the ratios was 3.6^5.5%. Samples were dried at 105‡C overnight and crushed. Approximately 0.1 g of each sample was dissolved using a CEM Corporation MDS 2000 Microwave Digestion System and concentrated trace metal grade HNO3 , HF, and HCl following EPA SW846 Method 3051. Samples were digested in Te£on reaction vessels using a combination of heat, pressure, and strong acids to dissolve the sediment. Once the digestion was complete, boric acid was added to stabilize the solutions. The samples were transferred to new
Fig. 2. Carbonate content of 1090 sediments based on Ca concentrations, the solid circles represent age control points based on oxygen isotope and magnetostratigraphy (Billups et al., submitted), the solid triangles represent age control points based on nannofossil datums (Marino and Flores, in press). The Eocene/Oligocene transition is shaded in gray. Several intervals of extremely low carbonate content are observed during the Oligocene; however, the dissolution event from 250 to 254 mcd occurs in the earliest Oligocene and can be correlated to dissolution events observed at other sites in the Southern Ocean. Galeotti et al. (in press) place the Eocene/Oligocene boundary at 270 mcd based on planktic foraminifera. We adopt the nannofossil-based biostratigraphy of Marino and Flores (in press) for this study.
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50-ml polypropylene centrifuge tubes and diluted to 50 ml with Milli-Q water. A Leeman Labs PS950 ICP^AES with a CETAC Corporation AT5000þ ultrasonic nebulizer was used to determine total elemental concentrations.
5. Results 5.1. Carbonate content Assuming carbonate is responsible for all Ca present, the weight percent of carbonate was calculated (Fig. 2). The calculated carbonate content is comparable to values obtained by standard coulometric techniques for carbonate content (Shipboard Scienti¢c Party, 1999). Using Ca concentrations as a proxy for carbonate appears not only to be valid, but also in this case yields a higher resolution record of carbonate content than is otherwise available. The preliminary Eocene/Oligocene boundary based on biostratigraphic reconstructions was placed at V240 mcd (Shipboard Scienti¢c Party, 1999). Our carbonate
155
record reveals a decrease in carbonate content in the interval from 250 to 254 mcd, with values dropping to near zero compared to about 10% carbonate before and after this interval. Based on these low carbonate values and the lack of preserved foraminifera, this interval likely represents a signi¢cant dissolution event. Similar dissolution events are observed at other sites throughout the Southern Ocean just after the Eocene/Oligocene boundary (Salamy and Zachos, 1999; Diester-Haass, 1996). Because of the proximity of this dissolution event to the biostratigraphic datums of Marino and Flores (in press) the dissolution event we observe in the interval from 250 to 254 mcd likely occurs at the beginning of the Oligocene. There are several other intervals of very low carbonate content likely re£ecting carbonate dissolution during the Oligocene. 5.2. Terrigenous content Using the Al (3113 Wmol g31 ) and Ti (110.7 Wmol g31 ) concentrations found in average crust (Taylor and McLennan, 1985), we normalized the
Fig. 3. Seven point smoothed percent terrigenous determined by Al normalization plotted against age. Gray shaded area represents the Eocene/Oligocene transition. The record is more variable during the Eocene and becomes more cyclic in the Oligocene and Miocene. Values exceeding 100% are artifact of the normalization process, meaning the end-member chosen for normalization purposes does not necessarily represent the actual end-member source.
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Al and Ti concentrations to calculate percent terrigenous material. Both calculations produce the same downcore trends; however, the calculated values are di¡erent. Many of the Ti-normalized values exceed 100%, whereas the Al values are typically lower. Al-normalized values are presented in Fig. 3. The hazard of normalizing geochemical values, of course, is that the average end-member source does not always represent the actual end-member source, which explains the calculated values greater than 100%. Values we calculated, however, vary in parallel with those interpreted from magnetic susceptibility and color re£ectance proxies, and also correspond to lower resolution visual shipboard assessments of percent terrigenous material. The Al/Ti ratio can be used to identify source types of detrital material, as di¡erent rock types have di¡erent Al/Ti ratios (e.g. Taylor and McLennan, 1985). The Al/Ti record is marked by two dominant modes: relatively higher and more variable values from 44 to 32.8 Ma, and lower and generally constant values from 32.8 to
16 Ma (Fig. 4). The Al/Ti ratio during the later constant interval averages 6.5 P 4.0, similar to a basaltic or oceanic crust value (V7.3; Taylor and McLennan, 1985). The Al/Ti ratio during the earlier interval averages 13.4, similar to the average continental crust value of 15.8, but has a high degree of variability ( P 19.0) re£ective of several extremely high values (Fig. 4). The highest rock end-member value for Al/Ti ratios is about 40 from granites (Taylor and McLennan, 1985), leading Murray and Leinen (1996) and Murray et al. (1993) to suggest that extremely high values found in equatorial Paci¢c sediments are the result of Al scavenging during intervals of high export productivity. Al/Ti ratios less than 40 may provide information about detrital sources, in particular whether sources are continental or oceanic, while Al/Ti ratios greater than 40 may provide information about export production. Excluding the extremely high values in the earlier interval, the Al/Ti ratio is generally higher and more variable, and undergoes a permanent shift at about 32.8 Ma.
Fig. 4. Seven point smoothed Al/Ti plotted versus age. The Al/Ti ratio is highly variable during the middle Eocene with maximum values higher than lithogenic sources. Granites have the highest Al/Ti ratio of greater than 40, which is exceeded during the middle Eocene. Late Eocene Al/Ti ratios are similar to those found in average continental crust (V15.8) or average upper crust (V27) suggesting a continental source (Taylor and McLennan, 1985). Values from about the Eocene/Oligocene boundary to 16 Ma are generally lower (Al/Ti = 10) and more similar to oceanic sources, such as oceanic crust or basalt (Taylor and McLennan, 1985).
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Fig. 5. Scatter plots of P and Ba concentrations versus Ti concentrations. A positive correlation with Ti suggests P and Ba are at least partially associated with terrigenous material.
and Delaney, 1994; Fo«llmi, 1995; Filippelli, 1997). At Site 1090, P is correlated with Ti, indicating that much of the P appears to be deposited in conjunction with terrigenous material (Fig. 5). This is di¡erent from other open ocean settings like the equatorial Paci¢c, where sedimentary P is linked quite strongly to biological processes (Filippelli and Delaney, 1996), but agrees with results from several other Southern Ocean sites (Latimer and Filippelli, 1997). A better proxy of non-terrigenous (i.e. biological) P is perhaps the P/Al and P/Ti ratios, which scale P as a function of terrigenous content. Records of P and P/Al and P/Ti ratios reveal some marked di¡erences in these proxies. For example, the long-term P record reveals an increase during the middle Eocene, a decline to just after the Eocene/Oligocene boundary, and apparent cyclic variations during the Oligocene and Miocene (Fig. 6, top). In contrast, the P/Al record reveals high values in the middle Eocene, with a peak at about 42 Ma corresponding to a peak in the bulk P record, then low and relatively constant values from the middle Eocene to the middle Miocene broken only by signi¢cant peaks from about 34.5 to 33 Ma (Fig. 6, middle). The P/Ti ratio also reveals high values in the earliest part of the record, followed by some peaks during the late Eocene/early Oligocene and a gradual increase through the Miocene (Fig. 6, bottom). In terms of a paleoproductivity proxy, we favor the P/Al record because (1) the variation in the P concentration record is likely also responding to variations in terrigenous matter input and is thus an imperfect proxy of paleoproductivity, and (2) the Ti record does not provide the proper normalization (as discussed earlier) and may have a less well constrained source than Al.
5.3. P records
5.4. Ba records
Phosphorus (P) is a limiting nutrient for productivity (Holland, 1978; Broecker, 1982; Smith, 1984), and is likely the most important on geologic time scales (Tyrell, 1999). For this reason, sedimentary records of P and P geochemistry have been used as a measure of paleoproductivity (e.g. Moody et al., 1988; Ingall et al., 1993; Filippelli
The accumulation of barite in sediments appears to be related to organic C export in surface waters; however, the exact relationship is not known (Dymond et al., 1992). Barium concentrations are a proxy for barite and have been considered as a productivity proxy (e.g. Lea and Boyle, 1990; Paytan et al., 1996; McManus et al.,
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Fig. 6. Seven point smoothed P concentrations (top), P/Al ratios (middle) and P/Ti ratios (bottom) plotted versus age. Because P is positively correlated with Ti, a terrigenously derived metal, we use P/Al and P/Ti ratios as proxies for export production rather than P concentrations. Most concentrations range from 20 to 65 Wmol P g31 , with elevated concentrations in the late early Miocene. Elevated P/Al and P/Ti ratios are interpreted as increases in export production. The most notable increases occur during the middle Eocene (V44^42 Ma) and across the Eocene/Oligocene transition (shaded area).
1998, 1999). Barium has a nutrient-type water column pro¢le with concentrations typically peaking below 2000 m. Preservation of Ba in sediments is dependent on several factors including bottom water oxygen and pore water sulfate concentrations, requiring the careful application and interpretation of Ba records as a productivity proxy (McManus et al., 1998). When pore water sulfate becomes depleted, barite is highly susceptible to dissolution ; however, downcore pore water sulfate pro¢les never go to zero at Site 1090 (Shipboard Scienti¢c Party, 1999). In younger, glacial/ interglacial sediments, Nu«rnberg et al. (1997) il-
lustrated that Ba is a valid productivity proxy in the south Atlantic sector of the Southern Ocean. Our Ba concentration record exhibits a sharp and permanent change at 33 Ma, from low concentrations (averaging 5 Wmol g31 ) with low variability before 33 Ma to high concentrations (averaging 21 Wmol g31 ) and relatively high variability after 33 Ma (Fig. 7, top). Similar to P, Ba concentrations are positively correlated with terrigenous material (r2 = 0.54). For this reason, Ba concentrations were normalized to terrigenous material by calculating Ba/Al and Ba/Ti ratios (Schroeder et al., 1997). Despite
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Fig. 7. Seven point smoothed Ba concentrations (top) Ba/Al ratios (middle) and Ba/Ti ratios (bottom) plotted versus age. Ba concentrations undergo a dramatic and permanent change at 32.8 Ma. Prior to 32.8 Ma, Ba concentrations are low, averaging 5 Wmol g31 , and after 32.8 Ma Ba concentrations are higher and more variable, averaging 21 Wmol g31 . Ba/Al and Ba/Ti ratios exhibit similar peaks to those seen in P/Al and P/Ti records (Fig. 6), with maxima from V44 to 42 Ma and across the Eocene/ Oligocene transition, which we interpret as increases in export production.
di¡erences between centrations, the P similar trends, with Eocene and across tion.
the total P and total Ba conratios and Ba ratios exhibit high values during the middle the Eocene/Oligocene transi-
6. Discussion 6.1. Variations in terrigenous/detrital material Increases in terrigenous supply have been suggested as a mechanism providing limiting trace nutrients to the surface ocean (Martin, 1990). A shift in the style of terrigenous sedimentation and
the geochemical composition of the terrigenous material occurs at Site 1090. Note that by ‘terrigenous’, we mean any input from a rock source, including terrestrial input, turbiditic input, and even hydrothermal and basaltic input. Clay content decreases during the middle to late Eocene, observed in visual and color re£ectance records as a decrease in sediment ‘redness’ but also determined by shipboard X-ray di¡raction analyses (Shipboard Scienti¢c Party, 1999). This decrease in clay content occurs despite a slight increase in terrigenous content. Thus, the relative content of the non-clay mineral component increases in this interval, possibly indicating a trend toward an increase in physically weathered mate-
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rial reaching this site and a decrease in the chemically weathered material. A similar change has been observed from sites much nearer to Antarctica, although the transition from chemical weathering-dominated to physically weathering-dominated terrigenous input occurs much later, from about 36 to 33 Ma (Ehrmann and Mackensen, 1992), than we observe here (from about 44 to 40 Ma). Beginning in the latest Eocene, terrigenous content becomes more cyclic compared to the highly variable terrigenous records prior to the Eocene/ Oligocene transition. In combination with the change in terrigenous content, a shift in the Al/ Ti ratio is observed. Excluding the very high Al/Ti ratios greater that 40, we still see a shift from continental sources prior to the Eocene/Oligocene transition to lower Al/Ti ratios with a likely oceanic source after the transition. This shift can only be interpreted as a source change from more continental crust to more oceanic crust detrital material at this site. The likely force driving this source change is deep-water circulation. An intriguing hypothesis is that 33 Ma marks the sudden onset of signi¢cant Paci¢c water bathing Site 1090. A change in deep current £ow such as this would bring entrained terrigenous material from the west, and this material was enriched in basaltic components (a likely scenario given rifting and ridge-building during the opening of the Drake Passage). We suggest, therefore, that signi¢cant ACC production began at 33 Ma, and remained strong and persistent from 33 to at least 16 Ma. 6.2. Export production variations A variety of proxies are used to assess productivity, including sedimentological (e.g. biogenic accumulation rates), biological (e.g. microfossil assemblages), and geochemical (e.g. carbon and nitrogen isotopes, biogenically enhanced trace metals, phosphorus) measures. Our approach here will focus on phosphorus, barium, and trace metal geochemistry, in the hope that these proxies can be integrated with other proxies at this site to develop a coherent picture of productivity variations through time.
Based on the P/Al and Ba/Al record, we identify two intervals of enhanced export production. The ¢rst interval occurs in the middle Eocene, from the base of our record at 44 Ma to approximately 42 Ma (Figs. 6 and 7). Sedimentologically, this interval is characterized by muddy nannofossil ooze with some chert, and contains a relatively high clay mineral content compared to the rest of the record. The P/Al ratios peak to above 1.0 (their highest values of the entire record), signi¢cantly above their average value of 0.1. The crustal value for P/Ti ratio is 0.01, and that found in carbonate rocks is 0.07 (Faure, 1999); the extremely high middle Eocene values thus indicate excess P burial and hence enhanced export production. This is further supported by elevated Ba/ Al and Ba/Ti ratios during these same intervals. This assertion is reinforced by the Al/Ti values during this interval, which peak to values well above any source other than is found during particulate scavenging and high export production (Murray et al., 1995). In fact, the P/Al ratios are small compared to what they could be, considering that the Al/Ti values indicate excess Al deposition that would normally drive down the P/Al ratio. High export production in the Southern Ocean during the middle Eocene is not without precedence: Diester-Haass and Zahn (1996) found peaks in paleoproductivity from about 44 to 41.6 Ma at the Maud Rise Site 689. A second high export production interval occurs from about 34.5 to 32.8 Ma (Fig. 6, middle and bottom). This event occurs as several discrete peaks in P/Al and P/Ti occurring before and after the Eocene/Oligocene boundary, coincident with peaks in P concentration (Fig. 6, top). Ba/Al and Ba/Ti ratios reveal similar discrete peaks across the Eocene/Oligocene transition further supporting short intervals of enhanced export production. Unlike the sharp peak in productivity proxies observed just after the Eocene/Oligocene boundary by Salamy and Zachos (1999), our results correspond more to the paleoproductivity record of Diester-Haass and Zahn (1996) from the Maud Rise, with similar sharp peaks beginning in the late Eocene. Based on these Subantarctic and Antarctic records, therefore, it appears that export production was indeed tem-
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porarily enhanced around the Eocene/Oligocene boundary, coincident with changes in oxygen isotopic composition re£ecting cooling and the onset of extensive Antarctic glaciation (e.g. Stott et al., 1992; Zachos et al., 1996). This enhancement, however, appears as several short (i.e. V100 000^200 000 yr) productivity peaks with a return to previous lower productivity conditions. Diester-Haass and Zahn (1996) attribute these transient increases in productivity to the ice build-up phenomenon, either strong oceanic overturn or the sudden input of dissolved nutrients during initial glaciation. Salamy and Zachos (1999) interpret the sharp productivity pulse just after the Eocene/Oligocene boundary to enhanced input of eolian material and possibly Fe-fertilization. Iron concentrations do increase during this interval (Fig. 8) coincident with P increases, but we know too little about the source of Fe and the relationship between Fe and P to brandish this as the smoking gun for the observed productivity increases.
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6.3. Ocean circulation variations The development of the ACC is clearly related to the opening of the Drake Passage, but much controversy exists about the style and timing of Drake Passage opening. For example, how wide and deep must the Drake Passage have been to allow adequate water £ux for a circum-polar current to develop and self-stabilize? Further, when did this occur? Several studies place the opening of the Drake Passage at about 30 Ma (Barker and Burrell, 1977; Lawver et al., 1992), while others note environmental and geochemical data indicating an earlier opening of about 37 Ma (e.g. Diester-Haass and Zahn, 1996). Still others suggest the opening of the Drake Passage occurred much later during the Oligocene near the Oligocene/Miocene boundary (Barker and Burrell, 1977). This is a topic of ongoing debate, as it is critical to understanding the thermal isolation of Antarctica and the global climatic response to that thermal isolation. For this reason, we wade into this
Fig. 8. Fe (solid triangles) and P (open circles) concentrations plotted against age. Salamy and Zachos (1999) suggested eolian Fe-fertilization increased productivity in the earliest Oligocene. From 34 to 32.8 Ma, Fe and P concentrations appear positively correlated, with peaks in both concentrations coincident; however, the exact relationship between Fe and productivity at this time is unknown.
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controversy with several observations from Site 1090. First, as mentioned before, the terrigenous records indicate a permanent shift in geochemical characteristics of the sediments at Site 1090 occurring at 32.8 Ma. We suggest that this marks the opening of the Drake Passage and the eastward transport of basaltic terrigenous material from the rifting regions to the west of Site 1090. Ba concentration records exhibit a permanent and dramatic shift at 32.8 Ma. We suggest that the Drake Passage was open to intermediate and deep waters at 32.8 Ma. We base this interpretation on the Ba records from this site. We suggest that this transition represents an adequate Drake Passage opening to allow for circulation of intermediate and deep waters with relatively higher preformed nutrient contents through the passage. This interpretation can be tested further by examining the same type of proxies in other Subantarctic, Subtropical, and Antarctic sites. One unknown, however, in this 32.8-Ma interpretation is that Ba is correlated to some degree with terrigenous matter (r2 = 0.54), and thus some of the Ba signal is simply a terrigenous signal (which we use to denote a 32.8-Ma opening of the Drake Passage). But the Ba/Ti record does reveal a unique character to the record before and after 32.8 Ma (Fig. 7, bottom), though not as clearly as the Ba record itself. Furthermore, the Ba/Al and Ba/Ti records re£ect several of the productivity maxima observed from other proxies in the middle Eocene and in the late Eocene/early Oligocene, further indicating its partial nutrient connection at this site.
7. Conclusions Geochemical proxies were used to construct high-resolution records of terrigenous content and export production at ODP Site 1090 for the middle Eocene through the early Miocene. Several interpretations arise from these results: (1) Terrigenous records indicate di¡erent styles and compositions of terrigenous material before and after 32.8 Ma. This change in sediment characteristics is supported by clay mineral content,
which suggests a change in dominance of chemically weathered to physically weathered material reaching the site. Al/Ti ratios prior to 32.8 Ma are more variable, relatively higher, and more similar to continental crust values, while Al/Ti ratios after 32.8 Ma remain constant and near ocean crust values. (2) Paleoproductivity proxies suggest two intervals of enhanced export production. Elevated P/Al ratios identify intervals of enhanced export production occurring from 44 to 42 Ma and from 34.5 to 32.8 Ma. Elevated Al/Ti ratios, perhaps indicative of particulate scavenging in the middle Eocene, further support increased export production at this time. Across the Eocene/Oligocene transition, export production appears to increase based on P and Ba ratios and concentrations. Both intervals of enhanced productivity correspond to records from other sites in the Southern Ocean. (3) The di¡erences in the style and characteristics of the geochemical records at 32.8 Ma suggest changes in deep-water circulation. Records of concentration and terrigenous content reveal a change from highly variable to more cyclic records beginning at the Eocene/Oligocene transition. The change in average Al/Ti ratios indicates a change in detrital source, from continental to oceanic crustal sources, that we suggest resulted from a change in deep-water circulation. Finally, Ba concentration records reveal a dramatic shift from low values to values that are highly variable and on average four times greater in magnitude than prior to 32.8 Ma. We suggest the Eocene/ Oligocene transition appears to mark the initiation of the ACC with the Drake Passage open to intermediate and deep waters by 32.8 Ma.
Acknowledgements We acknowledge research support from JOI, Inc. (Schlanger Ocean Drilling Fellowship to J.C.L.), JOI/USSSP (to G.M.F.), NSF (OCE 9711957 to G.M.F.) and the donors of the American Chemical Society through the Petroleum Research Fund. Comments from A. Bellanca, R. Gersonde, D. Hodell, R. Murray, C.
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