Middle Eocene to early Miocene environmental changes in the sub-Antarctic Southern Ocean: evidence from biogenic and terrigenous depositional patterns at ODP Site 1090

Global and Planetary Change 40 (2004) 295 – 313 www.elsevier.com/locate/gloplacha

Middle Eocene to early Miocene environmental changes in the sub-Antarctic Southern Ocean: evidence from biogenic and terrigenous depositional patterns at ODP Site 1090 Bernhard Diekmann a,*, Gerhard Kuhn b, Rainer Gersonde b, Andreas Mackensen b a

Alfred Wegener Institute for Polar and Marine Research, Research Unit Potsdam, Telegrafenberg A43, 14473 Potsdam, Germany b Alfred Wegener Institute for Polar and Marine Research, P.O. Box 12 01 61, 27515 Bremerhaven, Germany Accepted 16 September 2003

Abstract During Leg 177 of the Ocean Drilling Program (ODP), a well-preserved middle Eocene to lower Miocene sediment record was recovered at Site 1090 on the Agulhas Ridge in the Atlantic sector of the Southern Ocean. This new sediment record shows evidence of a hitherto unknown late Eocene opal pulse. Lithological variations, compositional data, mass-accumulation rates of biogenic and lithogenic sediment constituents, grain-size distributions, geochemistry, and clay mineralogy are used to gain insights into mid-Cenozoic environmental changes and to explore the circumstances of the late Eocene opal pulse in terms of reorganizations in ocean circulation. The base of the section is composed of middle Eocene nannofossil oozes mixed with red clays enriched in authigenic clinoptilolite and smectite, deposited at low sedimentation rates ( V 2 cm ka 1). It indicates reduced terrigenous sediment input and moderate biological productivity during this preglacial warm climatic stage. The basal strata are overlain by an extended succession (100 m, 4 cm ka 1) of biosiliceous oozes and muds, comprising the upper middle Eocene, the entire late Eocene, and the lowermost early Oligocene. The opal pulse occurred between 37.5 and 33.5 Ma and documents the development of upwelling cells along topographic highs, and the utilization of a marine nutrient- and silica reservoir established during the pre-late Eocene through enhanced submarine hydrothermal activity and the introduction of terrigenous solutions from chemical weathering on adjacent continents. This palaeoceanographic overturn probably was initiated through the onset of increased meridional ocean circulation, caused by the diversion of the Indian equatorial current to the south. The opal pulse was accompanied by increased influxes of terrigenous detritus from southern African sources (illite), mediated by enhanced ocean particle advection in response to modified ocean circulation. The opal pulse ended because of frontal shifts to the south around the Eocene/Oligocene boundary, possibly in response to the opening of the Drake Passage and the incipient establishment of the Antarctic Circumpolar Current. Condensed sediments and a hiatus within the early Oligocene part of the section possibly point to an invigoration of the deep-reaching Antarctic Circumpolar Current. The mid-Oligocene to lower Miocene section on long time scale exhibits less pronounced lithological variations than the older section and points to relatively stable palaeoceanographic conditions after the dramatic changes in the late Eocene to early Oligocene. D 2003 Elsevier B.V. All rights reserved. Keywords: Southern Ocean; Palaeoclimate; Eocene – Oligocene; Ocean circulation; Opal pulse; Terrigenous sediment

* Corresponding author. Tel.: +49-331-288-2170; fax: +49-331-288-2137. E-mail address: [email protected] (B. Diekmann). 0921-8181/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.gloplacha.2003.09.001

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1. Introduction In terms of Cenozoic climate and environmental history, the Eocene –Oligocene period has attracted palaeoclimatic research, as this time went along with climate deterioration, initial Antarctic glaciation, and the development of the circum-Antarctic Southern Ocean. From the present knowledge, inferred from terrestrial archives and sediment records on the Antarctic shelves, ephemeral ice masses possibly established during the late Eocene (Abreu and Anderson, 1998; Barker et al., 1999; Barrett, 1999). The Eocene/ Oligocene boundary marks an abrupt cooling step that was associated with the first significant establishment of permanent Antarctic ice sheets and enhanced formation of cool deep-water masses along Antarctica (Lear et al., 2000; Zachos et al., 2001).

Changes in palaeocenography were moreover mediated through reorganizations in southern hemispheric ocean circulation. Since the late Eocene the diversion of the Indian equatorial current through the progressive northward motion of the Indian plate and the restriction of Mediterranean basins gave rise to meridional current patterns, which affected the current systems of the southern Indian Ocean and the southeastern South Atlantic (Lawver and Gahagan, 1998). At the earliest since the early Oligocene, continental drift led to the opening of deep sea conduits between Antarctica and South America (Drake Passage) and between Antarctica and Australia (Tasmanian Gateway), promoting the thermal isolation of Antarctica (Lawver and Gahagan, 1998; Barker, 2001; Exon et al., 2001). The exact timing of the latter tectonic processes, however, still is a matter of controversial debate.

Fig. 1. The Southern Ocean with the present-day position of the Polar Front Zone (Whitworth, 1988) and locations of ODP Site 1090 and other DSDP and ODP Sites referred to in the text.

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Important archives of Eocene – Oligocene environmental change occur in the form of marine deposits encountered around Antarctica in the Southern Ocean that bear environmental signals recorded by the biogenic and terrigenous sediment components (Kennett and Barker, 1990; Barron et al., 1991; Ehrmann and Mackensen, 1992; Diester-Haass et al., 1993; Lazarus and Caulet, 1993; Thomas and Gooday, 1996). During Leg 177 of the Ocean Drilling Program (ODP), a well-preserved and almost complete and undisturbed succession of middle Eocene to lower Miocene marine deposits was recovered at Site 1090 in the southeastern Atlantic sector of the subAntarctic Southern Ocean (Gersonde et al., 1999) (Fig. 1). As a unique feature, which so far was not recognized in its dimension in the Southern Ocean, it includes a f 100-m-thick succession of upper middle Eocene to lower Oligocene diatomaceous oozes, which apparently is linked with late Eocene climate cooling prior to the onset of extended Antarctic glaciation. This observation is of particular interest, because biosiliceous blooms also punctuated global cooling events and changes in ocean circulation during former periods in earth history, as during the late Devonian and at the Cretaceous/Tertiary boundary (Racki, 1999). Another example is a late Pliocene opal pulse in the Benguela upwelling area off southwestern Africa, which preceded the late Cenozoic cooling step (Lange et al., 1999; Berger et al., 2002). The goals of this paper are to gain new insights into environmental changes during the middle Eocene to early Miocene and to explore the circumstances of the late Eocene opal pulse in the subAntarctic South Atlantic in terms of climate change and reorganizations in ocean circulation. Our interpretations are based on the survey of the depositional environment and compositional variations of the biogenic and lithogenic sediment fractions of the Site 1090 record.

2. Material and methods Site 1090 was drilled at 3710 m water depth in a small sedimentary basin on the southwestern part of the Agulhas Ridge (42j54.81VS, 08j53.98VE) (Gersonde et al., 1999) (Fig. 1). The sediment infill overlies

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upper Cretaceous crustal basement (Raymond and LaBreque, 1988). Today, Site 1090 is situated in the Antarctic Circumpolar Current north of the Polar Front Zone (Whitworth, 1988). A total of 162 samples was taken at F 1.5-m intervals from the spliced section between 85 and 241 m composite depth (mcd), encountered in Holes 1090B, 1090D, and 1090E, and from the section of Hole 1090B down to 409 mcd. In addition, 137 samples, originally used for ship-board physical-properties measurements, were integrated in our sample set, but were only analysed for bulk sediment parameters. Temporal resolution ranges between 50 and 200 ka for the middle Eocene sediments and the Oligocene to lower Miocene sediments, and ranges below 50 ka in the opal-rich upper Eocene sediments. Sample specifications and data lists can be extracted from the data bank system PANGAEA via the Internet (http://www.pangaea.de). Determination of bulk sediment parameters, comprising percentages of organic carbon, biogenic carbonate, and biogenic opal, and the calculation of respective mass-accumulation rates (MAR), followed the procedures also applied for the Pleistocene section of Site 1090, including determination of organic and inorganic carbon through loss-on-ignition measurements and opal determination by an automated leaching method (Diekmann and Kuhn, 2002). MARs of individual components were calculated by multiplying linear sedimentation rates with values of dry-bulk density and the proportions of the respective sediment fractions. Values of dry-bulk density were taken from the shipboard data set of Site 1090 (Gersonde et al., 1999). We refer to the lithogenic fraction as the nonbiogenic sediment fraction. Values of sedimentation rates and MARs were resampled to equal time increments of 250 ka through integration between original data points. This procedure smoothens artificial spikes in sedimentation rates, resulting from the uneven distribution of age marker. On selected bulk samples, major elements (without Si) and the trace elements Cu, Ni, and Zn were analysed by inductively coupled plasma emission spectroscopy with relative analytical confidence of F 5%. Acid digestions of the samples were performed in Teflon beakers using a mixture of ultrapure HNO3, HClO4, and HF. Stable oxygen isotope measurements of the total carbonate fraction were determined with a Finnigan

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MAT 251 isotope ratio gas mass spectrometer, directly coupled to an automated Kiel-1 carbonate preparation device. Isotope values are referenced to the Vienna Pee Dee Belemnite standard and are given in y-notation with an analytical precision better than F 0.08x . The carbonate-free sediment fraction < 63 Am was measured with a ‘Micromeritics SediGraph 5000E’ to infer the grain-size distribution of lithogenic silt and coarse clay in 1/10-Phi steps. However, these grainsize distributions may be overprinted by grain-size signals from biogenic opal in the carbonate-free fraction. Because of very robust biosiliceous particles, traditional opal-leaching methods with NaOH and Na2CO3 solutions failed. Instead, biogenic opal was removed from silt samples by density separation in a centrifuge with sodium metatungstate solution (D = 2.30 g cm 3), after the silt fraction had been separated from the clay fraction in settling tubes at 2 Am. Density separation of opal is not applicable on the

clay fraction, because of high cohesion between the clay particles. Clay mineralogy was inferred from X-ray diffraction measurements of glycolated preferentially oriented clay mounts. Sample preparation and evaluation of X-ray diffractograms followed standard techniques described elsewhere (Ehrmann et al., 1992; Petschick et al., 1996).

3. Stratigraphy The applied age model (Fig. 2) is based on the palaeomagnetic record of Site 1090 (Gersonde et al., 1999; Billups et al., 2002), which was correlated with the geomagnetic polarity time scale (Cande and Kent, 1995; Berggren et al., 1995). The age model has been corroborated by stable isotope data for the late Oligocene to early Miocene interval (Billups et al., 2002) and by biostratigraphic datums of calcar-

Fig. 2. Age – depth relationship at ODP Site 1090. Chronostratigraphic interpretation refers to the geomagnetic polarity time scale (Cande and Kent, 1995; Berggren et al., 1995). Used age model for the late Oligocene to early Miocene is based on palaeomagnetic interpretations and stable isotope stratigraphy (Billups et al., 2002). Palaeomagnetic interpretations of the older section were inferred from biostratigraphic datums of calcareous nannofossils (Marino and Flores, 2002) and planktonic foraminfera (Galeotti et al., 2002) as well as the occurrence of diagnostic impact ejecta (Kyte, 2001). The position of Chron 13n was constrained in this study by the identification of the Oi-1 isotope stage.

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eous nannofossils (Marino and Flores, 2002) and planktonic foraminifera (Galeotti et al., 2002) for the older part of the section. An important age control point (35.5 F 0.2 Ma) is provided by a sediment layer with abundant upper Eocene impact deposits at 391 mcd (Kyte, 2001), constraining palaeomagnetic Chron 16n1. Because of uncertainties regarding the exact depth level of the Eocene/Oligocene boundary (Galeotti et al., 2002; Marino and Flores, 2002), we conducted stable oxygen isotope measurements on bulk sediments between 290 and 220 mcd, to identify the Oi-1 oxygen isotope shift associated with palaeomagnetic Chron 13n (Zachos et al., 1996). Because this core interval is almost barren of foraminifera, we had to rely on the isotope signature of total carbonate, which nonetheless displays the Oi-1 stage above 241 mcd (Fig. 2). At 221 mcd, an unconformity and a hiatus were identified for the time span between 32.8 and 31.3 Ma

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on the basis of calcareous nannofossil stratigraphy (Marino and Flores, 2002), which also is indicated by disrupted sediments in core 1090B-23X-7 below 90 cm. The best evidence for a hiatus is provided by seismic records from the site location that display a marked unconformity at the respective depth level (Wildeboer Schut et al., 2002).

4. Results At Site 1090, variations in lithology and sediment composition reflect changes in both biogenic and lithogenic depositional patterns that developed through distinct time intervals (Figs. 3 –7). Apparent sedimentation rates, inferred form the age – depth relationship, and MARs of individual biogenic and lithogenic sediment components document the combined effects of sediment fluxes and the rates of deposition, preservation, and burial.

Fig. 3. Temporal variations of the proportions of major sediment components at ODP Site 1090. Geochemical data (P/Al ratios) were taken from Latimer and Filippelli (2002), using the refined age model introduced in this article.

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4.1. Lithology and sedimentation rates Pale nannofossil oozes and chalks mixed with reddish clay, which are almost barren of biosiliceous remains, characterize the middle Eocene section between 43.3 and 38.9 Ma (Gersonde et al., 1999). Initially, the succession was deposited at moderate sedimentation rates (1.7 cm ka 1) that decreased after 41 Ma (0.9 cm ka 1) (Fig. 7). The upper middle Eocene to lowermost Oligocene section between 38.9 and 33.4 Ma is mostly represented by an extended succession (118 m thickness) of grey diatomaceous oozes and muds with occasional calcareous layers (Gersonde et al., 1999). Some horizons within the opal-rich sediments between 37.5 and 33.5 Ma exhibit faint laminations, similar to diatom mats present in Pleistocene sections from Sites 1093 and 1094, which were recovered south of the Polar

Front Zone from sediments of the Circum-Antarctic Opal Belt (Gersonde et al., 1999) (Fig. 1). The presence of an 18 cm thick chert layer at 353 mcd (38.9 Ma), which forms the lower boundary of the interval, and another 5 cm thick chert layer at 288 mcd (35.2 Ma) indicate diagenetic processes acting between pore water and the opal-rich host sediment (Fig. 3). Sedimentation rates were comparably high (0.9 –3.8 cm ka 1), reaching two maxima around 37 Ma and just below the Eocene/Oligocene boundary (34.0 Ma) (Fig. 7). Assuming that the two chert layers represent diatomaceous oozes that were reduced to about 10% of their original thickness by diagenetic processes (Fu¨chtbauer, 1988), the calculated sedimentation rates seem to be slightly underestimated. However, the two chert layers correspond to 2.30 cm oozeequivalent, which is negligible in relation to the great thickness of the diatomaceous ooze succession.

Fig. 4. Temporal grain-size variations of carbonate-free sediment (center) and carbonate-/opal-free silt (to the right) at ODP Site 1090.

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The lower Oligocene section between 33.4 and 30.2 Ma consists of pale grey calcareous oozes and muds (Gersonde et al., 1999). The section represents a condensed sediment succession that was deposited at low sedimentation rates ( < 0.5 cm ka 1) and includes a hiatus between approximately 32.8 and 31.3 Ma (Fig. 7). A change to reddish-brown sediment colour marks the youngest interval of the investigated section (Gersonde et al., 1999). It generally consists of sediments enriched in lithogenic components, with transitional alternations between opal-rich and more calcareous muds (Fig. 4). The interval was deposited at moderate sedimentation rates between 0.8 and 1.9 cm ka 1, with maximum values through the late Oligocene.

vertical shifts of the lysocline with its effects on carbonate preservation, or (3) intrastratal dissolution due to the degradation of organic matter and the related release of organic acids and CO2. The relative importance of these processes is difficult to asses with confidence from our data. Enhanced calcareous export production very likely played an important role for carbonate deposition prior to 40.5 Ma, as supported by phosphorus and barium data of the Site 1090 sediment record (Latimer and Filippelli, 2002) (Fig. 3). In contrast, dissolution effects apparently occurred in the late Eocene biosiliceous oozes, as indicated by the poor preservation of foraminifera (Galeotti et al., 2002).

4.2. Biogenic carbonate

Above the diatom-barren middle Eocene section, proportion of biogenic opal varies between 40% and 80% in the upper middle Eocene to lowermost Oligocene section between 38.9 and 33.4 Ma (Fig. 3). Opal MARs reach peak values (0.2 to 0.7 g cm 1 ka 1) with maxima coeval with maxima in sedimentation rates (Fig. 7). These values are even greater than those in the Pleistocene section of Site 1090, which vary between 0.05 and 0.3 g cm 1 ka 1 (Diekmann and Kuhn, 2002). The appearance of diatom mats is consistent with elevated MARs, because they document intense phytoplankton blooms in the surface water and a rapid sinking of the organic remnants to the sea floor (Kemp and Baldauf, 1993). As observed in the present-day frontal system of the Antarctic Circumpolar Current (Smetacek, 1999), the upwelling of nutrient-rich water masses probably favoured seasonal phytoplankton blooms during the late Eocene. Increased biological productivity fluxes is also suggested by high P/Al ratios in the Site 1090 record (Fig. 3), showing a maximum between 35 and 33 Ma around the Eocene/Oligocene boundary (Latimer and Filippelli, 2002). Low concentrations in biogenic opal ( V 16%) and low opal MARs ( < 0.13 g cm 1 ka 1) characterize the lower Oligocene section between 33.4 and 30.2 Ma. Opal percentages increase again in the remaining Oligocene and lower Miocene section (15 – 50%), but MARs of opal (0.1 – 0.3 g cm 1 ka 1) generally range below the maxima reached during the late Eocene (Fig. 7).

Throughout the studied section, carbonate concentrations exhibit pronounced variability (0 –85%) with marked short-term fluctuations at high-amplitude (Fig. 3). The long-term trends in carbonate deposition are displayed by changes in carbonate MARs, integrated at 0.25-Ma time steps (Fig. 7). Maximum values up to 1.2 g cm 1 ka 1 occurred during the middle Eocene, between 43.0 and 41.5 Ma, while they decreased ( < 0.7 g cm 1 ka 1) during the following time. Apart from a few calcareous layers that contain up to 50% carbonate, carbonate concentrations do not exceed 20% in the upper middle Eocene to lowermost Oligocene section between 38.9 and 33.4 Ma and carbonate MARs range between low and moderate values (0.10 – 0.67 g cm 1 ka 1). Carbonate percentages in the lower Oligocene section between 33.4 and 30.2 Ma mostly range above 45% with moderate MARs at the base (0.57 g cm 1 ka 1) that decline to low values ( < 0.05 g cm 1 ka 1) below and above the hiatus and slightly increase toward the top of the section (0.23 g cm 1 ka 1). Carbonate contents (0– 80%) show a highly variable pattern with carbonatebarren intervals around the early to late Oligocene boundary between 30 and 28 Ma, in the upper Oligocene section between 25.9 and 25.0 Ma, and in the lower Miocene section between 23 and 21 Ma. The recorded variability of carbonate concentrations and MARs may reflect the relative importance of (1) calcareous vs. siliceous biological production, (2)

4.3. Biogenic opal

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4.4. Organic carbon Concentrations of organic carbon are low ( < 0.08%) through the studied section (Fig. 3). Organic carbon variability is positively associated with variations in the percentage of biogenic opal. Higher concentrations of organic carbon (0.15%), however, appear in the upper part of the opal-rich upper Eocene section (Fig. 3). 4.5. Lithogenic sediment fraction Percentages of the lithogenic sediment fraction show high variability through the entire section, depending on the proportions of biogenic constituents. MARs of lithogenic sediment mostly range above 0.2 g cm 1 ka 1 with maxima in the upper Eocene (up to 1.2 g cm 1 ka 1) in conjunction with strong opal accumulation and in the upper Oligocene (0.5 g cm 1 ka 1). During the late Eocene, in addition to high lithogenic fluxes, enhanced vertical

export of biosiliceous particles may have encouraged the rapid settling of fine-grained lithogenic particles by scavenging and aggradation effects in the water column. 4.6. Grain-size distribution of silt and clay Grain-size distribution in the carbonate-free sediment fraction is extremely fine-grained in the middle Eocene section, consisting of up to 80% fine-sized clay particles < 0.5Am in diameter (Fig. 4). A gradual coarsening towards higher silt concentrations is evident in the upper middle Eocene section between 41 and 38 Ma. Grain-size distributions with similar amounts of silt and clay persist through the upper Eocene to lower Miocene section. These grain-size patterns are also displayed by the grain-size distributions of the carbonate- and opal-free sediment fraction (Fig. 4), showing that the grain-size signal of the lithogenic sediment fraction is not significantly overprinted by the grain-size signal of biogenic opal.

Fig. 5. Temporal variations of clay-mineral parameters at ODP Site 1090.

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4.7. Clay mineralogy In the middle Eocene section, the high abundance of fine clay is consistent with a clay-mineral assemblage, which is dominated by well crystallized smectite ( F 90%) with minor illite and traces of kaolinite (Fig. 5). The mineral spectrum also includes clinoptilolite, which was identified as dispersed authigenic zeolite crystals during ship-board studies of smear slides (Gersonde et al., 1999). The younger section ( < 38.9 Ma) is dominated by poorly crystallized smectite and does not include zeolite. A continuous increase in illite, kaolinite and chlorite is evident in

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the middle to upper Eocene section between 38.9 and 35 Ma. Abundant terrigenous clay minerals ( F 50%) characterize the upper Oligocene section, coinciding with high MARs of lithogenic sediment (Figs. 5 and 7). A change towards higher kaolinite/chlorite ratios occurred after 27.0 Ma and indicates a stronger relative supply of kaolinite in respect to the other terrigenous clay minerals, illite and chlorite. 4.8. Geochemistry The occurrence of clinoptilolite together with the presence of well crystallized smectite in the middle

Table 1 Results of geochemical analyses (weight proportions) of selected Site 1090 bulk samples (upper part) in comparison with the geochemistry of typical marine deposits (lower part, from Chester, 1990) Elements

Diat. Calc. mud, 249.46 mcd (33.74 Ma)

Diat. mud, 341.16 mcd (37.55 Ma)

Calc. ooze, 380.31 mcd (41.67 Ma)

Calc. clay, 381.17 mcd (41.72 Ma)

Calc. ooze, 407.06 mcd (43.22 Ma)

Al2O3 (%) CaO (%) Fe2O3 (%) K2O (%) MgO (%) MnO (%) Na2O (%) P2O5 (%) TiO2 (%) Fe (ppm) Mn (ppm) Cu (ppm) Ni (ppm) Zn (ppm) Mn/Fea Cu/Fea Ni/Fea Zn/Fea

7.00 17.79 3.44 1.67 1.77 0.22 3.00 0.24 0.61 24,056 1704 25 37 75 70.80 1.04 1.54 3.12

9.87 6.21 5.01 1.79 2.47 0.26 3.83 0.33 0.89 35,035 1998 38 60 101 57.00 1.08 1.71 2.88

1.76 50.48 0.78 0.42 0.48 0.13 0.96 0.13 0.08 5427 1038 19 22 29 191.30 3.50 4.05 5.34

8.63 24.68 4.13 1.84 2.01 0.26 1.75 0.28 0.41 28,881 1975 97 96 96 68.40 3.36 3.32 3.32

1.26 52.31 0.54 0.35 0.35 0.11 0.88 0.17 0.06 3783 868 n.d. 12 55 229.30 n.d. 3.17 14.54

Elements

Near-shore mud

Deep-sea carbonate

Atlantic deep-sea clay

Pacific deep deep-sea red clay

Ridge sediments

Manganese nodules

Fe (ppm) Mn (ppm) Cu (ppm) Ni (ppm) Zn (ppm) Mn/Fea Cu/Fea Ni/Fea Zn/Fea

69,900 850 48 55 95 12.20 0.69 0.79 1.36

9000 1000 30 30 35 111.10 3.33 3.33 3.89

82,000 4000 130 79 130 48.80 1.59 0.96 1.59

65,000 12,500 570 293 n.d. 192.30 8.77 4.51 n.d.

180,000 60,000 730 430 380 333.30 4.06 2.39 2.11

140,580 220,000 3300 5700 3500 1564.90 23.47 40.55 24.90

a

Element ratios  1000. Abbreviations: Diat.: diatomaceous, Calc.: calcareous, n.d.: not determined.

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Eocene section possibly points to authigenic processes, often associated with the deposition of red clays (e.g. Chester, 1990). The availability of elements needed for the neoformation of minerals mostly results from the dissolution of opal and unstable silicates in combination with element scavenging from the water column (Cole and Shaw, 1983; Na¨hr et al., 1998). To test this assumption, we have conducted geochemical analyses of selected samples from the middle Eocene section and overlying sediments and compared with the geochemical signal of detrital and hydrogenous marine deposits (Table 1). In comparison with detrital marine sediments, red clays and ridge sediments are usually enriched in total iron, and are strongly enriched in Mn and the trace elements Cu, Ni, and Zn (e.g. Chester, 1990). The analysed samples from Site 1090 show a marked variability in their major- and trace-element distributions (Table 1), apparently depending on the dilution effects by biogenic elements (Ca, and Si which was not determined). Nevertheless, the presence of authigenic elements in the middle Eocene sediments is suggested by increased Mn/Fe, Cu/Fe, Ni/Fe, and Zn/ Fe ratios. Similar differences in element ratios also appear between the standard detrital and hydrogenous marine deposits referred to in Table 1.

5. Discussion The basic aspects of the Site 1090 sediment record comprise: (1) a transition from a depositional setting with prevailing biogenic carbonate production towards a regime with biosiliceous producers by the end of the middle Eocene; (2) the new regime beginning with a pronounced biogenic opal pulse in the late Eocene; (3) a progressive increase in the contribution of terrigenous detritus from the late Eocene to early Miocene. 5.1. The preglacial middle Eocene Southern Ocean After the early Eocene climate optimum (Zachos et al., 2001), calcareous oozes/chalks with a dominance of calcareous nannofossils (Coccolithophoridae) indicate a warm- to temperate mode of biological productivity with reduced nutrients in the surface waters at Site 1090, according to the lithological criteria used

by Burckle et al. (1996). This finding is consistent with warm to temperate sea-surface temperatures in the middle to high southern latitudes, as also revealed by oxygen isotope records of benthic and planktonic foraminfera (Zachos et al., 2001; Oberha¨nsli, 1996) and the palaeoecological signals of planktonic foraminifera at Site 1090 (Galeotti, 2002). It also fits the pattern of dominant carbonate deposition in most parts of the Southern Ocean at this time (Lazarus and Caulet, 1993). Increased carbonate MARs together with geochemical proxy data (Latimer and Filippelli, 2002) reflect enhanced biological productivity prior to 42 Ma, which is also evident in the Site 689 sediment record from Maud Rise near Antarctica (DiesterHaass and Zahn, 1996). Another environmental aspect was a low supply of terrigenous matter and the dominance of smectite in the non-biogenic sediments of the Southern Ocean and adjoining South Atlantic, derived from pedogenic terrestrial sources (Ehrmann and Mackensen, 1992; Robert and Chamley, 1992). At Site 1090, the presence of abundant clinoptilolite, well-crystallized smectite and hydrogenous trace elements moreover points to authigenic hydrogenous processes associated with the deposition of red clays in association with the calcareous deposits. Smectite –clinoptilolite associations were also reported from Maud Rise (DiesterHaass et al., 1993) and are globally widespread in Cretaceous to Eocene calcarous oozes and clays, deposited at low sedimentation rates (Kastner and Stonecipher, 1978; Lisitzina and Butuzova, 1982). Required cations and silica for smectite and zeolite formation might have been provided via intrastratal solution or directly from sea-water. In this context, the progressive dilution of hydrothermal plumes and the dispersal of elements and authigenic particulates in seawater is one factor promoting a hydrogenous environment in the ocean away from active ridges and vents (Hillier, 1995; Chamley, 1997). The latter possibility seems to have been important, because the last Cenozoic maximum in global submarine magmatic activity and in the production of ocean crust and hydrothermal plumes occurred during the middle Eocene and declined afterwards (Owen and Rea, 1985; Engebretson et al., 1992; Berner, 1994; Courtillot et al., 2003). A local influence on hydrogenous sedimentation during the middle Eocene can be ruled out, because a former spreading ridge in the vicinity

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of Site 1090 was abandoned and displaced about 825 km to the west along the transform faults of the Agulhas Fracture Zone some 65 million years ago (LaBrecque and Hayes, 1979). Another potential source of elements was the influx of terrestrial silica-rich weathering solutions from southern Africa and Australia as a result of extensive continental silcrete formation (Summerfield, 1983; McGowran, 1989; Wopfner and Walther, 1999). 5.2. The late Eocene opal pulse By the end of the middle Eocene, a pronounced environmental change took place at Site 1090, documented by an opal pulse as an indicator of increased phytoplankton production and stronger upwelling. At the same time, marine temperatures declined (Oberha¨nsli, 1996; Lear et al., 2000; Zachos et al., 2001; Billups and Schrag, 2003) and water column stratification became less developed (Stott et al., 1990). Cosmopolitan planktonic biota evolved to more endemic populations (Lazarus and Caulet, 1993) and benthic foraminifera became more specialized in exploiting phytodetritus (Thomas and Gooday, 1996). The opal pulse took place, when widespread opal accumulation (mainly cherts) declined in the North Atlantic and the equatorial seas (Baldauf and Barron, 1990; Berger, 1991). Other areas in the Southern Ocean that experienced systematic increases in biosiliceous sedimentation during the late Eocene were the Falkland Plateau (DSDP Site 511) (Ludwig et al., 1980) and the western South Tasman Rise (ODP Site 1170) (Exon et al., 2001). However, at Site 1171 nearby on south Tasman Rise, late Eocene diatom remains overlay cherts of middle Eocene age, and at Site 1172 on the eastern Tasman Plateau, diatom remains go back to the middle Eocene around 42 Ma (Exon et al., 2001). Closer to Antarctica, on Maud Rise (Site 689) and on the Kerguelen Plateau (ODP Sites 738 and 744), increased opal accumulation also started in the late Eocene, but reached its maximum in the earliest Oligocene (Ehrmann and Mackensen, 1992; Salamy and Zachos, 1999). On Maud Rise, the timing of maximum opal deposition is consistent with a synchronous peak in cold-mode biological productivity, inferred from stable isotope records and abundances of benthic foraminifera on Maud Rise (Diester-Haass and Zahn,

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1996). On the Kerguelen Plateau, the early Oligocene opal MARs reached 0.3 g cm 1 ka 1 (Salamy and Zachos, 1999) and thus ranged below the late Eocene peak values of the Site 1090 record (0.7 g cm 1 ka 1) (Fig. 7). Thus it seems that enhanced opal deposition was a diachronous event and was most pronounced in the Atlantic sector of the Southern Ocean. It started, and was more intense, in the northern part (37.5 –33.5 Ma) and then switched to the south (34.0 –30.5 Ma). A common feature of all locations is the association of enhanced opal accumulation with topographic highs, which apparently promoted vertical movements of water masses. The opal pulse was related to palaeoceanographic changes that favoured upwelling, perhaps in response to tectonically forced reorganizations in global ocean circulation. In this context, one important aspect was the development of circumAntarctic circulation through the establishment of ocean gateways between Antarctica and South America (Drake Passage) and between Australia and Antarctica (Tasmanian Gateway). Modeling studies examining the potential sedimentary responses to changes in ocean circulation through the opening and closure of ocean gateways, actually predict a depression of the lysocline and enhanced opal deposition in the South Atlantic after the clearance of the Drake Passage (Heinze and Crowley, 1997). This sedimentary response is consistent with the Site 1090 sediment record, but would require an unimpeded deep-water flow through the Drake Passage during the late Eocene. The timing of the Drake Passage shallow and deep opening, however, is still controversial. Based on plate-tectonic reconstructions circum-Antarctic circulation between the Pacific Ocean and the South Atlantic was shallow during the Paleogene and Eocene, while deep communication was not initiated earlier than 32.5 Ma and became established around 30 Ma in the early Oligocene (Lawver and Gahagan, 1998). Latest findings of the latter authors suggest an open Drake Passage that was cleared to deep-water circulation by 31 F 2 Ma (Lawver et al., 2003). Other plate-tectonic reconstructions suggest that a deep-water pathway through the Drake Passage was created in the Miocene within the period 22 to 17 Ma (Barker, 2001), preceded by the opening of shallow passages between 29.7 and 21.8 Ma (Eagles and

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Livermore, 2002). Geochemical parameters of the Site 1090 sedimentary record point to a stronger supply of detrital matter with affinities to ocean crust composition since 32.8 Ma (indicated by decreasing Al/Ti ratios), possibly provided by the bathing of the Agulhas Ridge with Pacific water masses (Latimer and Filippelli, 2002). In contrast, Nd isotope data indicate that Site 1090 was not influenced by radiogenic Pacific water during the early Oligocene and that the majority of deep opening between the Pacific Ocean and the South Atlantic occurred between 28.5 and 23 Ma (Scher and Martin, 2001). From variations in Miocene phytoplankton growth rates in the South Atlantic, the opening of Drake Passage to deep-water flow was fully established around 20 Ma (Pagani et al., 2000). Despite these conflictive estimates, it becomes evident that the opal pulse started much earlier (37.5 Ma) than the Drake Passage had opened to deeperlevel flow sometime between 31 and 17 Ma. The opal pulse also started before a deep conduit through the Tasmanian Gateway was established around 33.5 Ma, as recently revealed by sediment records recovered on the South Tasman Rise (Exon et al., 2001). The initiation of the late Eocene opal pulse on the Agulhas Ridge can be alternatively related to the diversion of the Indian equatorial current around the southern tip of Africa, as a result of progressive northward motion of the Indian plate and restriction of the Mediterranean basin around 40 Ma (Baldauf and Barron, 1990; Oberha¨nsli, 1996; Lawver and Gahagan, 1998). Following the latter authors, we assume that the Southern Ocean from that time was connected to meridional current patterns, implying equatorial heat loss, change of marine and atmospheric temperature gradients with effects on the wind systems, and moisture transfer to the southern high latitudes. These environmental changes were important prerequisites for later Antarctic glaciation and for a more vigorous ocean circulation with the development of upwelling cells and fronts. The onset of late Eocene upwelling eventually gave rise to the biological utilization of dissolved silica and nutrients, derived from hydrothermal activity and terrigenous input of weathering solutions that were enriched in the sluggish pre-late Eocene Southern Ocean (Owen and Rea, 1985; McGowran, 1989; Wopfner and Walther, 1999). The late Eocene opal

pulse therefore can be regarded as the realization of the postulated ‘‘silica burp’’ in the Eocene ocean (McGowran, 1989). The end of the late Eocene opal pulse in the earliest Oligocene, as recorded at Site 1090, may be related to frontal shifts in conjunction with the development of the proto-Antarctic Circumpolar Current. This process possibly moved the locus of prominent opal deposition closer to the Antarctic margin (e.g. Maud Rise and Kerguelen Plateau). Another aspect of the opal pulse is its correspondence to maxima in Corg concentrations at Site 1090. This observation supports the assertion that, in contrast to the tropical Eocene seas, opal deposition in the Southern Ocean coincided with increased burial rates of organic carbon and therefore with a removal of CO2 from the ocean surface and atmosphere (Berger, 1991). It can be speculated that this type of biological pump may have contributed to the consumption of atmospheric CO2 as a positive feedback mechanism on climate deterioration during the late Eocene. However, the role of atmospheric CO2 on Eocene climate change still is a matter of controversial debate (Freeman and Hayes, 1992; Pearson and Palmer, 1999). And it is questionable whether regional depositional processes in the study area were sufficient to affect the global CO2 budget. The late Eocene opal pulse shows striking similarities to the circumstances of the early Matuyama Diatom Maximum, a late Pliocene opal pulse that took place between 3.1 and 2.0 Ma in the Benguela upwelling area off southwestern Africa (Lange et al., 1999; Berger et al., 2002). Apart from the different durations of the pulses (4 Ma in the late Eocene vs. 0.9 Ma in the late Pliocene), both events were associated with planetary cooling and preceded strong increases in global ice volume. In both instances, the strengthening of upwelling gave rise to the removal of silicate from the upper ocean that was inherited from a previous warmer ocean; and both pulses were characterized by southward diachronism and frontal shifts. The late Eocene opal pulse in the sub-Antarctic ocean was followed by an early Oligocene opal pulse off Antarctica, while the early Matuyama Diatom maxima off southwestern Africa, which is also evident in the sub-Antarctic South Atlantic (Diekmann et al., 2003), was followed by an early Pleistocene opal pulse in the Polar Front Zone (Gersonde et al., 1999; Berger et al., 2002; Diekmann

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et al., 2003). This comparison underscores the palaeoenvironmental significance of the late Eocene opal pulse. 5.3. The early Oligocene current event The late Eocene opal pulse on the Agulhas Ridge was followed by a period of reduced sediment accumulation between 33.4 and 30.2 Ma, and a 1.5-Ma hiatus around 32 Ma that also was identified as an pronounced unconformity in seismic profiles of the Agulhas Ridge (Wildeboer Schut et al., 2002). This hiatus is not only of local significance, because unconformities in the lower Oligocene and around the lower to upper Oligocene boundary can be traced throughout the Southern Ocean (Tucholke and Embley, 1984; Wright and Miller, 1993; Fulthorpe et al., 1996). The unconformities demonstrate the invigoration of bottom currents in the early Oligocene. The onset of condensed sedimentation at Site 1090 corresponds to the Oi-1 stable isotope stage and thus apparently occurred in conjunction with the intensification of dense and corrosive bottom-water formation in East Antarctica, as a consequence of early Oligocene ice-sheet expansion (Mackensen and Ehrmann, 1992; Zachos et al., 1996). However, high carbonate concentrations (mostly >45%) in the condensed lower Oligocene section (Fig. 3) argue against a strong influence of corrosive water masses at Site 1090. Therefore, the current event may indicate a general strengthening of abyssal circumpolar circulation consistent with the ongoing widening and deepening of the Drake Passage, which culminated about 30 my ago (Lawver and Gahagan, 1998). This second possibility, however, suffers from the controversies concerning the exact timing of Drake Passage opening to deep-water flow (see Section 5.2). 5.4. Enhancement of terrigenous sediment input The late Eocene opal pulse was accompanied by a progressive increase in the input of terrigenous detritus, indicated by increases in MARs of lithogenic sediment (Fig. 7) and an increase in silt-sized siliciclastic particles (Fig. 4). It is best displayed by changes in the clay-mineral spectrum with concomitant increases in the pure terrigenous clay minerals

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illite, chlorite, and kaolinite at the expense of smectite (Fig. 5). As pointed out, smectite to a great extent seems to be of marine origin. The enhanced supply of the pure terrigenous clay minerals during the late Eocene to early Oligocene at the expense of smectite, observed at Site 1090, is also evident at many other locations in the Atlantic and Indian sectors of the Southern Ocean (Ehrmann and Mackensen, 1992; Robert and Chamley, 1992). In general, it can be explained by the global cooling trend that favoured stronger mechanical weathering of source rocks and soils in the continental source areas, enhanced aeolian dust supply, glacigenic sediment input from Antarctica, and stronger particle advection in ocean currents. Another likely effect was sea-level lowering that reduced the storage of terrigenous detritus on the continental shelves. However, the influence of the various processes showed regional and temporal differences. Increased glacigenic input in response to ice-sheet extension mainly controlled clay-mineral assemblages off East Antarctica (Ehrmann and Mackensen, 1992; Robert and Chamley, 1992). In that region, proportions of terrigenous clay minerals did not increase significantly before the early Oligocene (Fig. 7). In contrast, proportions of terrigenous clay minerals were already elevated during the late Eocene at Site 1090 and also farther north, for example on the Walvis Ridge (Robert and Chamley, 1992). This diachronous pattern suggests different sources and modes of transportation of clay minerals between the various locations (Fig. 7). Potential sources can be inferred in a ternary diagram (Fig. 6), in which proportions of the terrigenous clay minerals illite, chlorite, and kaolinite of the Site 1090 record display an assemblage clearly dominated by illite. In the modern South Atlantic and the adjoining Atlantic sector of the Southern Ocean, such an assemblage only appears in marine sediments along the East Antarctic coast, reflecting the dominance of high-grade metamorphic rocks in the glacial source areas, and along the southern African coast, where it is derived from the dry areas of the continental hinterland (e.g. Petschick et al., 1996; Diekmann et al., 2003). These sources of illite-rich material were also evident in late Eocene –Oligocene times near East Antarctica and off southern Africa (Robert and Chamley, 1992). Today, the supply of

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Fig. 6. Clay-mineral proportions of the ODP Site 1090 sediment record in a ternary diagram of the terrigenous clay minerals illite, kaolinite, and chlorite (black dots represent measurements of the individual Site 1090 samples). The grey discrimination fields and their assignments represent modern clay-mineral assemblages found in sea-bottom sediments of the South Atlantic and adjoining Southern Ocean (modified from Diekmann et al., in press). Ruling out East Antarctica (see Discussion), the observed illite-dominated assemblage of Site 1090 points to southern African terrigenous sediment sources.

illite from East Antarctica to the north is inhibited by the modern ocean circulation system of the Southern Ocean (Petschick et al., 1996), and apparently did not play a great role during late Eocene – Oligocene time, because enhanced illite supply at Site 1090 started earlier than in East Antarctica. Assuming southern Africa as the major illite source, another conclusion is that a dry climate with dominant physical weathering existed in southern Africa, at least since the late Eocene. During the late Oligocene, a slight increase in kaolinite/chlorite ratios at Site 1090 might reflect increased continental humidity and chemical weathering after 26 Ma (Fig. 5), as also indicated in many other clay-mineral records of the world oceans (Robert and Chamley, 1987). Although no comparable terrestrial climate records from southern Africa are available (Partridge, 1993), findings from marine records off southwestern Africa and from the Walvis Ridge basically support our interpretation of climate conditions (Dingle and Hendey, 1984; Robert and Chamley, 1984).

Two modes of transportation can be invoked to achieve clay-mineral supply to Site 1090, aeolian input and particle advection in ocean currents. Glacial supply can be neglected, because the Site 1090 record apparently does not include ice-rafted detritus. General circulation models, as during the Quaternary, indicate a setting of Site 1090 within the westerly wind zone already for the Eocene (e.g. Sloan and Huber, 2001). Therefore, direct aeolian supply to Site 1090, situated southwest of southern Africa, possibly was insignificant. Leaving aquatic transport as the best option, the southern African source signal points to particle supply within a southwestward-directed ocean current. Today, such a particle transport is evident for the Agulhas Retroflection that moves Indian Ocean waters to the southeastern South Atlantic (Goldstein et al., 1999; Kuhn and Diekmann, 2002). Therefore, we postulate a proto-Agulhas Current during the late Eocene– Oligocene, originating form a western boundary current in the western Indian Ocean that provided sediment supply. This interpretation from clay mineralogy, in addition to

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Fig. 7. Temporal variations of sedimentation rates and mass-accumulation rates (MAR) of major sediment components at ODP Site 1090. Opal record of the Kerguelen Plateau (filled black curve) refers to ODP Site 744 (Salamy and Zachos, 1999) and clay-mineral trend (shaded curve) on the Kerguelen Plateau and on Maud Rise refers to ODP Sites 689, 690, 738, and 744 (Ehrmann and Mackensen, 1992). Right column shows the stacked global benthic foraminiferal d18O record as a proxy of global environmental change (Zachos et al., 2001). Note the diachronism of opal and clay-mineral records between ODP Site 1090 and the Antarctic ODP Sites in respect to the Oi-1 stage.

the interpretation of the opal pulse, gives another clue for the reorganization of global ocean circulation since the late Eocene, triggered by the diversion of the Indian equatorial current. 5.5. Depositional patterns after the late Eocene opal pulse The results demonstrate that sedimentation during the Oligocene to early Miocene period was not affected by dramatic changes in the depositional facies at Site 1090, but switched in a transitional manner between episodes of stronger and weaker input of terrigenous matter and the accumulation of biogenic matter. Biogenic opal forms an integral

constituent of the Oligocene to Miocene marine sediments at Site 1090, but was less dominant than during the late Eocene opal pulse (Fig. 7). By the end of the opal pulse, the Eocene nutrient and silica reservoir at Site 1090 possibly was reduced and depositional patterns adjusted to the new environmental boundary conditions. Carbonate MARs generally ranged below those recorded in the middle Eocene interval and temporarily dropped to near zero, suggesting periods with severe dissolution between 30 and 28 Ma, around 26 Ma, and around 22 Ma (Fig. 7). The dissolution periods may reflect shallowing of the lysocline in response to the variable spread of corrosive bottom waters produced along the glaciated Antarctic margin

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(Mackensen and Ehrmann, 1992; Zachos et al., 1996). The verification of these dissolution events, however, requires a closer look on carbonate-preservation proxies in future. Since the end of the middle Eocene, a moderate to high supply of terrigenous matter reflected the general impact of global climate deterioration on detrital fluxes (see Section 5.4). In conclusion, the post-Eocene era heralded the climate-controlled depositional patterns of the modern ice ages in the northern part of the Southern Ocean, although marked contrasts to modern conditions still existed. Sediments deposited in the Oligocene to Miocene ocean had lower abundances of planktonic foraminifera in relation to nannofossils, had reddish colours, and lacked coarse-grained icerafted detritus (Gersonde et al., 1999). Modern glacial –interglacial depositional patterns, implying the formation of the Circum-Antarctic Opal Belt south of the Polar Front Zone, were first established during the late Pliocene (2.4 Ma) with increasing northernhemispheric glaciation and its marked impacts on the global environment (Hodell and Ciesielski, 1990; Diekmann et al., 2003). This palaeoceanographic development in the Southern Ocean highlights the unique occurrence of the late Eocene opal episode, anticipating the deposition of the Pleistocene biosiliceous oozes.

6. Conclusions The Site 1090 sedimentary record from the southeastern Atlantic sector of the sub-Antarctic Southern Ocean provides a new archive of Eocene –Oligocene climate and environmental change. The studied section shows that the late Eocene apparently was a turning point in the Cenozoic depositional history with subsequent increased deposition of biosiliceous remains, reduced carbonate production and/or deepsea preservation and a higher terrigenous sediment input in the course of global climate cooling. An important finding is the recognition of a marked late Eocene opal pulse, which started in the latest middle Eocene (37.5 Ma) and ended in the earliest Oligocene (33.5 Ma). This pulse may have been associated with enhanced burial of organic matter with possible effects on atmospheric CO2 drawdown and climate deterioration, prior to the first significant

expansion of Antarctic ice sheets at the Eocene/Oligocene boundary. Changes of the depositional environment can be attributed to long-term tectonic processes that led to reorganizations in ocean circulation and palaeoceanography. The opal pulse may be related to the closing of Tethyan seaways and the diversion of the proto-Indian Ocean equatorial current to the south by the end of the middle Eocene. The resulting western boundary currents in the Indian Ocean possibly entered the southeastern South Atlantic around the southern tip of Africa. The inflow of Indian Ocean waters is indicated by illite-rich detrital material originating from southern African sources. This event gave rise to the development of frontal systems and upwelling cells, promoting strong opaline export production after 37.5 Ma at Site 1090. The opal pulse likely ended with the incipient development of the Antarctic Circumpolar Current after 33 Ma in the early Oligocene, as a result of the ongoing opening of the Drake Passage and the Tasmanian Gateway. This process possibly led to frontal movements and shifted the locus of prominent opal deposition towards Antarctica. At the same time, bottom-water circulation became stronger, as evidenced by reduced sediment accumulation between 33.4 and 30.2 Ma, and a 1.5-Ma hiatus around 32 Ma. In outlook, the late Eocene opal pulse reveals a linkage to an important global cooling step and deserves further palaeoclimatic attention, when studying the Cenozoic marine-geological record.

Acknowledgements This research used samples and data provided by the Ocean Drilling Program, which is sponsored by the U.S. National Science Foundation and participating countries under management of the Joint Oceanographic Institutions. Funding for this study was provided by the Deutsche Forschungsgemeinschaft through grant Di-655/2. We thank the organizers of the Ocean Drilling Program for inviting B.D., G.K., and R.G. to participate Leg 177 and we appreciate the kind assistance of the ‘Joides Resolution’ ship crew and scientists. The paper and its earlier drafts benefited from the constructive reviews of Kenneth

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Miller, Dave Hodell, Christian Robert, Claus-Dieter Hillenbrand and one anonymous reviewer. This is publication awi-n11245 of the Alfred Wegener Institute for Polar and Marine Research.

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