Paleoproductivity of the Indian Ocean during the Tertiary Period

GLGBAL fWGNETARY

ELSEVIER

Global and Planetary

Change 11 (1995) 71-88

Paleoproductivity of the Indian Ocean during the Tertiary Period William G. Siesser Department of Geology, Vanderbilt University, Nashuille, TN 3723, USA Received 20 September

1994; accepted after revision 24 February 1995

Abstract Studies of oceanic paleoproductivity have lagged in part because of the absence of a suitable proxy for measuring paleoproductivity. A transfer function has recently been proposed which directly translates carbonate mass accumulation rates into a quantitative measure of productivity. This transfer function has been applied at six ODP sites in the Indian Ocean in order to investigate temporal and spatial changes in Indian Ocean productivity during the Tertiary Period. Early Paleogene productivity in the Indian Ocean was considerably higher than in the late Paleogene and Neogene. A warmer, more confined sea, with large riverine nutrient input from close lying land masses may account for the generally higher early Paleogene productivity. Productivity increased dramatically at two times during the Paleogene: during early Paleocene calcareous nannofossil Zone NP5 and during early Eocene Zone NPll. These productivity increases may be

related to abrupt climatic changes which reorganized oceanic circulation and stimulated major changes in productivity. Productivity began to fall in the Oligocene as a result of global cooling and the initiation of the modem Indian Ocean circulation system. This trend continued into the Neogene, which was characterized by generally interspersed with occasionally higher levels, especially at the more northern sites in the Indian Ocean.

1. Introduction Primary productivity has been defined as “...the amount of inorganic carbon converted into organic carbon by photosynthesis (“fixed”) per unit of ocean area per unit of time” (Ryther, 1969; Thompson, 1978). Recent interest in productivity involves the influence of phytoplankton on global change. It has been recognized that calcareous plankton remove vast amounts of carbon from the atmosphere via the oceans every year, rivaling the amount extracted by tropical rain forests (Winter and Briano, 1989). Variations in oceanic productivity are thought by some (see Broecker, 1982; Sundquist and Broecker, 1985; Keir, 1988) to be responsible for much of the ob0921-8181/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0921-8181(95)00003-8

low productivity,

served fluctuation in atmospheric CO, recorded in polar ice during the last 150,000 years (Barnola et al., 1987; Raynaud et al., 1992). Also, the recent confirmation that coccolithophores (major primary producers) are the chief emitter of dimethyl sulfide (Keller, 1989; Keller et al., 1989), which may act to generate oceanic cloud cover (Charlson et al., 1987; Andreae, 19901, has been heralded as suggesting partial biologic regulation of Earth’s climate (Charlson et al., 1987). It is clear that an improved knowledge of both modem productivity and paleoproductivity in the oceans is important in understanding the global carbon system and its present and past effects on the Earth (e.g. Pisias, 1991). The chief primary producers in the oceans today

72

W.G. Siesser / Global and Planetary Change I I (1995) 71-88

are the diatoms, coccolithophores ’, dinoflagellates, and picoplankton (Westbroek et al., 1989; Kleijne et al., 1989); other groups are minor contributors. The same primary producers are assumed to have been dominant producers in the oceans since Cretaceous time. As heterotrophs, planktic foraminifers are secondary producers, but are also important to the measurement of productivity in this study. The strongest influences controlling modern primary productivity are oceanic fertility (nutrient supply) and solar irradiation. Secondary and/or related controls are oceanic circulation, surface-water stability, turbidity, cloud cover, and temperature (Fogg, 1975; Margalef, 1978,1985; Thompson, 1978; Hallock, 1987; Legendre, 1990; Reid et al., 1990; Hallock et al., 1991). The same controls, modified by long-term forcing conditions such as changes in climate, are also assumed to have regulated productivity in the past. The goal of this study is to examine changes in paleoproductivity, specifically productivity generated by the calcareous microplankton, in the open-ocean regions of the Indian Ocean during the Tertiary Period. Carbonate mass-accumulation rates (MAR) will be the proxy indicator used to assess paleoproductivity. Major questions to be addressed in this investigation are: (1) How did productivity change on a temporal basis during the Tertiary Period in the Indian Ocean? (2) What was the spatial distribution pattern of productivity in the Indian Ocean during different Tertiary time-slices, and how does the Tertiary distribution correspond to spatial distribution of productivity in the modem Indian Ocean?

2. Study area The Indian Ocean was chosen for this study primarily because of the continuity of many of the Tertiary sections obtained during recent drilling in

-0

Fig. 1. Location map showing ODP sites investigated. Site 709 on Madingley Rise; 758 on northern part of Ninetyeast Ridge; 757 on central Ninetyeast Ridge; 762 on Exmouth Plateau; 752 on Broken Ridge; 748 on Kerguelen Plateau.

that ocean. The Indian Ocean has been drilled extensively during DSDP and ODP cruises. ODP operations have used the Advanced Piston Corer (APC) for coring as much as possible because this tool allows almost complete core recovery. After termination of APC coring at a site, the Extended Core Barrel (XCB) is used, which also allows fairly good recovery. Only ODP sites were selected for this study, owing to their better core recovery. Selection of specific ODP sites was further constrained by geographic location within the ocean, water depth, stratigraphic completeness of the Tertiary section, and nannofossil preservation. Wide spatial coverage of the Indian Ocean was desired. Six sites were chosen: 709, 748, 752, 757, 758, and 762, which extend from 5” 23’N to 58” 27’S, and from the Madingley Rise (709) in the west to the Exmouth Plateau (762) in the east (Fig. 1).

3. Productivity proxies and the oligotrophic ocean -i----[In this study, “coccolithophores”

is used when specifically referring to the living organisms, “calcareous nannofossils” is used when specifically referring to fossil forms, and “calcareous nannoplankton” is used as the general collective term for the group, living and fossil (Siesser and Haq, 19871.1

A persistent problem in investigating paleoproductivity has been the selection of a reliable proxy, or set of proxies, to measure ancient primary productivity. A variety of proxies have been suggested

W.G. Siesser / Global and Planetary Change 1 I (1995) 71-88

and/or used, including: accumulation rates of organic carbon (e.g. Diester-Haass, 1978; Muller and Suess, 1979; Siesser, 1978,198O; Bralower and Thierstein, 1984,1987; Samthein et al., 1987; Stein et al., 1989a,b; Thierstein, 1989), accumulation rates of biogenic silica and/or biogenic carbonate (e.g. Berger, 1976; Diester-Haass, 1978; Siesser, 1978,1980,1993; Samthein et al., 1982; Stein, 1985; Barron and Baldauf, 1989; Herbert et al., 1989; Boersma and Mikkelsen, 1990; Silver and Gowing, 1991; Froelich et al., 1991), diversity (e.g. Tappan and Loeblich, 1973; Roth, 1979; Roth and Bowdler, 1981; Watkins, 1989; Siesser, 1993), trace elements (e.g. Bishop, 1988; Bruland et al., 1989; Andrews et al., 1990; Elderfield, 1990; Siesser et al., 1992) carbon isotopes (e.g. Delaney, 1989; Dudley and Nelson, 1989; Herbert et al., 1989; Oberhansli and Von Salis Perch-Nielsen, 1990), and alkenone abundance (e.g. Brassell et al., 1986; Prahl et al., 1988; Prahl and Muehlhausen, 1989; Marlowe et al., 1990). Less attention has been given to investigations of paleoproductivity in low-productivity, open-ocean areas than in higher-productivity, nearshore areas (Muller and Suess, 1979; Sarnthein et al., 1987; Brummer and Van Eijden, 1992). Productivity is higher in nearshore areas because of the greater supply of nutrients and the resulting larger standing crop of plankton there (Tappan, 1968). For the modern ocean, Pinet (1992) gave values of < 137 mg C m p2 day-’ for typical open-ocean areas, 137-411 mg C mm2 day-’ for continental shelves, and 4111370 mg C mm2 day-’ for upwelling areas. Despite their relatively low productivity, open-ocean areas are of major importance in the global carbon cycle because their low productivity per given area is offset by the huge geographic areas involved, and thus the very large amount of total carbon produced in oligotrophic regions (Tappan, 1968). Studies of oligotrophic areas have also lagged in part because few of the proxies described earlier are readily applicable to open-ocean sediments (Brummer and Van Eijden, 1992; Van der Zwaan et al., 1992). Organiccarbon accumulation rates, for example, are considered to be a direct indicator of primary productivity (Diester-Haass, 1978; Stein et al., 1989a,b; Van der Zwaan et al., 1992). Organic carbon is, however, readily oxidized during and after transportation to the ocean floor, and only a small portion of the

73

organic carbon manufactured in surface waters is preserved in the underlying sediments. In the openocean environment, which includes most of the ocean, organic carbon is so efficiently oxidized that only about 0.03% of the primary production is eventually incorporated in sea-floor sediments (Brummer and Van Eijden, 1992). The extremely small amount of organic matter available is thus unsuitable for studies of most open-ocean sediments. In this study the carbonate accumulation rate is used as an indirect proxy of paleoproductivity for the open-ocean regions of the Indian Ocean. It should be stressed that this study deals with productivity as generated by calcareous nannoplankton and planktic foraminifers in oligotrophic, open-ocean conditions. No extrapolations can be made concerning total productivity, or productivity in nearshore/upwelling zones.

4. Carbonate

mass accumulation

rates

Calcareous nannoplankton are the major primary producers in oligotrophic, open-ocean areas (Smayda, 1980; Aubry, 1992). Calcareous nannoplankton may bloom in truly extraordinary numbers when temperature, nutrient, and other environmental conditions are optimal. Examples of such periodic high nannoplankton productivity are well known (e.g. Holligan et al., 1983). Planktic foraminifers also increase in abundance under the same conditions, but their increase is in large part a response to the increased abundance of autotrophs, other heterotrophs, and organic detritus on which they feed. The total flux of foraminifers and calcareous nannoplankton skeletons to the sea floor certainly increases as the productivity of these organisms rises (McIntyre and McIntyre, 1971; Diester-Haass, 1978; Siesser, 1978,198O; Silver and Gowing, 1991). Until recently, however, the biogenic carbonate accumulation proxy lacked a transfer function which could directly translate microfossil skeletons into a quantitative measure of paleoproductivity, i.e. in terms of carbon production in mg C me2 day-‘. Brummer and Van Eijden (1992) have recently proposed such a transfer function, and it is used in this study. The rationale behind the transfer function is that pelagic CaCO, production in the open ocean is

74

W.G. Siesser / Global and Planetary Change 11 (1995) 71-88

proportional to organic carbon production, as shown by Goldman (1980) and Broecker and Peng (1982). Investigations of data from open-ocean sediment traps have also demonstrated a high correlation between carbonate and organic carbon fluxes (Honjo, 1980; Deusser, 1987; Deusser and Ross, 1989). Brummer and Van Eijden (1992) sum up the rationale well when they state “... bulk carbonate fluxes generated by primary and secondary producers together do correlate very highly with organic carbon fluxes in open ocean environments where biogenic carbonate predominates over biogenic silica (generally the blue ocean...). Since organic carbon fluxes are generally considered to reflect primary productivity, this must also be the case for carbonate fluxes”. It is assumed that the relationship between carbonate and organic carbon production has not changed significantly during the Tertiary. For purposes of calculation, all carbonate is assumed to be essentially pelagic in origin. The rare benthic foraminifers present do not significantly affect the calculations. 4.1. Methods The bulk sediment accumulation rate for a given stratigraphic interval was calculated by multiplying the linear sedimentation rate (LSR) for the interval (m/m.y.) by the average dry-bulk density (DBD) of the sediment samples in that interval (g/cm3>. Averages based on approximately 560 samples were used in this study. Use of dry-bulk density values provided an accumulation rate corrected for sediment compaction (Van Andel et al., 1975). The sediment accumulation rate was then multiplied by the weight percent CaCO, and recalculated to obtain the carbonate MAR in mg rn-* day-‘. Data on DBD, wet-bulk density (WBD), porosity, water content, and CaCO, percent for ODP core samples are given in the ODP Initial Reports volume for each site, with the exception of DBD values for Site 762, which were not published. DBD values for Site 762 were calculated from WBD and porosity data following the method of Van Andel et al. (1975). In summary: Carbonate

UAR

= Wt% - CaCO,/lOO

X

LSR X DBD

(1)

Brummer and Van Eijden (1992) introduced a correction factor for carbonate dissolution (1.47-see next section), and their equation for calculating paleoflux of carbonate (Fcaco,) thus becomes: F CaCO,= 1.47 (Carbonate

MAR)

(2)

Data on the organic carbon to carbonate flux ratio have been obtained from a large number of sediment-trap studies (e.g. Honjo, 1980,1984; Reynolds and Thunell, 1985; Deusser, 1987; Deusser and Ross, 1989; Wefer, 1989; Jickells et al., 1990). From these studies and other data, Brummer and Van Eijden (1992) calculated an averaged organic carbon to carbonate flux ratio of 0.725 + 0.2 for open-ocean environments. Organic carbon paleoflux ( FC_org)becomes: F c erg= FCC0 x 0.725 x 0.12 3

(3)

where 0.12 is the molar weight ratio of carbon to CaCO,. The final step is to obtain an estimated value for primary productivity in the open ocean by using the equation of Betzer et al. (1984), as modified and rewritten by Brummer and Van Eijden (19921, to relate primary paleoproduction (PpP) in the euphotic open ocean to export production represented by the paleoflux of organic carbon. Thus: PpP=

2.527(F,_0,g)‘1~7’ X (depth in meters)“‘s’s (4)

This equation includes a multiplication factor of 1.3406 in order to convert to mg C me2 day ‘. Brummer and Van Eijden (1992) tested their method by using their equations to calculate Holocene productivity at different locations, then compared their results to productivity values for the same locations based on other data. The results agreed well enough to suggest that their transfer function provides a reasonable estimate of primary productivity. The accuracy of the method must not be accepted uncritically, however, as several potential sources of error can affect the calculated estimate of carbonate MAR, and thus of paleoproductivity. Dissolution, winnowing and redeposition, hiatuses, and inaccuracies in the sedimentation-rate calculations can all affect the productivity values obtained. These

W.G.Siesser/ Globaland PlanetaryChange 11 (I 995) 71-88

potential sources of error, and their effects, are discussed in the following paragraphs (see also Brummer and Van Eijden, 1992). 4.2. Dissolution Significant amounts of carbonate dissolution would invalidate results obtained by the application of the Brummer and Van Eijden (1992) transfer function. Samples chosen for this study are therefore from locations that are above the Calcite Compensation Depth (CCD) today, and have been since deposition (Van Andel, 1975). Van Andel (1975) summarized the Cenozoic history of the CCD in the Indian Ocean as follows: During the Eocene the CCD was at about 3600 m. The CCD began to deepen in the Indian Ocean during the late Eocene, gradually falling to a depth of about 4400 m by the late Oligocene. Early in the Miocene the CCD rose to about 4000 m, then fell sharply in the late Miocene to about 5200 m, where it is today. All the sites chosen for this study are above the CCD (the deepest site is 3041 m), and have been throughout the Tertiary (Van Andel, 1975). The sites have also probably not been below the lysocline (presently at 3900 m; Brummer and Van Eijden, 1992) during the Tertiary, as shown by the degree of preservation of the samples. Nevertheless, the number of calcareous skeletons buried will always be less than that produced in surface waters because of some dissolution. Corroded foraminiferal tests and removal of certain dissolution-prone species (both foraminifers and nannofossils) are indicators of sediments significantly affected by solution. Only sample intervals with records of no effects, or of only minimal effects of dissolution were selected for calculations in this study. Most of the aragonite settling to the ocean floor is dissolved and is never incorporated in the sediment (Berner and Honjo, 1981; Morse and Mackenzie, 1990). Although calcite is more resistant to solution than aragonite, some of it is also dissolved prior to burial, even above the CCD (Berger, 1976; Adelseck, 1978; Yang et al., 1990). Brummer and Van Eijden (1992) compiled estimates showing that of the original microfossil carbonate flux above the lysocline, 15% is still lost owing to aragonite dissolution and 20% owing to calcite dissolution. The correction factor (1.47) described earlier has thus been introduced into

the equation for calculating carbonate (Brummer and Van Eijden, 1992).

75

MAR

4.3. Winnowing and redeposition Winnowing by currents selectively removes fine material from the sediment and significant winnowing can introduce an error in MAR calculations. Brummer and Van Eijden (1992) calculated a correction factor for several samples they suspected of being affected by winnowing. The best way to avoid this potential source of error, and to obviate the need for correction factors, is to use only samples that have not been winnowed. Relative absence of fine sediment, a relative abundance of large, poorly preserved planktic foraminifers, and an abnormally high benthic/planktic foraminiferal ratio are indicators of winnowing (Brummer and Van Eijden, 1992). Descriptions of samples used in this study were examined for any indications of winnowing, and only samples from unwinnowed or essentially unwinnowed intervals were used. A small amount of redeposited material is almost always present in a given sample of marine sediment. Nannofossil biostratigraphers know that if a slide is searched long enough, at least a few specimens will probably be found that are out of place stratigraphically (reworked from older sediment). These are normally insignificant in terms of the volume of the sediment. A large number of such specimens would, however, indicate an unacceptable level of redeposition. Again, selection of sample intervals was made after careful inspection of the sediment descriptions, in order to avoid error caused by redeposition. 4.4. Hiatuses Pitfalls in the calculation of the LSR may be caused by hiatuses, especially when the duration of the hiatus is uncertain. Brummer and Van Eijden (1992) extrapolated across several hiatuses in their study, but note the uncertainty of such calculations. Stratigraphic completeness was one of the criteria used in selecting the sites to be investigated here; nevertheless, some hiatuses exist in the sites selected. In this study, MARS were not calculated for

W.G. Siesser / Global and Planetary Change I1 (1995) 71-88

76

intervals where whole or partial are missing. 4.5. Linear sedimentation

nannofossil

The calcareous nannofossil biozonation of each core was reviewed to confirm the zonal boundary assignments. Martini’s (1971) calcareous nannofossil zonation is used throughout this study. The ages of Martini’s (1971) zonal boundaries have recently been recalculated by Wei and Peleo-Alampay (1993), using a much improved geomagnetic polarity time scale published by Cande and Kent (1992). This new scale permits more accurate LSR calculations than

zones

rates

The MAR value obtained is very sensitive to accurate LSR determinations, which in turn are crititally dependent on the placement of biozonal boundaries, and on the accuracy of the time scale used.

550 -500 --

0,

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19 16. 15 12 11 10 9 8-6 5 4-l 24- 23 22 21 1E 16. 15 14 13 12 11 10 9 &7 18 1s

25

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e

Miocene

P/i0

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;

5 4-3 2

;

NP Zones

NN Zones I

: 6

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._I 650 600

04:

::

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19 16 1s 12 11 10 9 8-6 5 4-l 24- 23 22 21 1.3 16- 15 14 13 12 11 10 9 8-7 6 25 20 17 18 1s

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I e

INNZ~MI~S I me/ I

P/i0

Miocene

I

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~NPZmes 1 e Oligo

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m Eocene

Fig. 2. Temporal changes in Tertiary productivity. Graphs of mg C me2 daycalcareous nannofossil zones of Martini (1971). a. Site 709. b. Site 758.

I Paleocene

e ,

’ (mean values for each zonal interval) plotted against

W.G. Siesser / Global and Planetary Change II (1995) 71-88

77

those made in the past. For example, two of the sites used in this study (752 and 757) were also investigated by Brummer and Van Eijden (1992). Recalculation of the i%%Rat those sites, using the new zonal ages of Wei and Peleo-Alampay (19931, gave MAR values different from those of Brummer and Van Eijden (19921, who used the Berggren et al. (1985) time scale for their calculations.

of Cenozoic circulation based on ocean General Circulation Model results. Using these maps, the current-system patterns for each sub-epoch were roughly approximated and the circulation information used during interpretation of the productivity results.

4.6. Results

5.1. Site 709

Paleoproductivity results for the Indian Ocean are given in the next sections in two forms. The first is a series of temporal plots (Figs. 2-4), showing productivity fluctuations at each site with time. Productivity values shown on the plots are mean values calculated from several determinations within each nannofossil zone. Although this study concentrates on the Tertiary, a value for Pleistocene Zone NN19 is also included in several plots in order to compare productivity during this nearest-present time with productivity values in the Indian Ocean today. The second form of presentation is a series of maps (Figs. 5-10) showing spatial distribution of paleoproductivity in the open-ocean areas of the Indian Ocean for each sub-epoch of the Tertiary. These maps were constructed to show the geography of the Indian Ocean at the middle of each sub-epoch, interpolated from the plate reconstructions of Scotese et al. (1988) and Scotese (1990). Productivity ranges displayed on the maps represent mean values calculated from numerous determinations within each sub-epoch. Where productivity values are not shown for a sub-epoch or a zone it is because hiatuses, poor preservation, or winnowing would render potentially inaccurate results for that interval. Consideration of the changing paleoceanography of.the Indian Ocean during the Tertiary is important in interpreting the productivity results. As shown by the maps in Berger (1989) and Berger and Wefer (19911, waters above the individual sites vary considerably in their productivity today. The position of each site with respect to shifting current systems and convergence/divergence positions during different Tertiary time intervals will affect productivity at that site. Haq (1984) published a series of maps showing ocean circulation during various Tertiary time intervals. Barron and Peterson (1991) also presented maps

This site is located on the Madingley Rise in the western equatorial Indian Ocean. ODP drilling at this site terminated in middle Eocene sediments. Fig. 2a shows that the average productivity during the middle Eocene to late Oligocene (NP16-NP25) at this site fluctuates within a range between 89 and 231 mg C mm2 day-‘. Productivity for the early Miocene (NNl-NN 5) interval was not calculated because of sediment reworking and the presence of a hiatus. Productivity was low during the middle Miocene (NN6-8), rose during the late Miocene (NNlO-NNll), then dropped again briefly at the Miocene-Pliocene boundary (NN12). Productivity rose sharply again during the early Pliocene (NN1315), then decreased slightly and leveled off during the late Pliocene (NN16-18) and Pleistocene (NN19) to a mean value of 177 mg C m-’ day-‘. This is within the present productivity range of surface waters in the region of Site 709 today: 164-274 mg C m -2 day- ’ (recalculated from productivity maps in Berger and Wefer, 1991).

5. Temporal productivity

5.2. Site 758 Site 758 lies on the northern part of the Ninetyeast Ridge. Productivity is low (45 mg C rnd2 day-‘) in the first measured zone of the early Paleocene (NP3-41, but rises sharply to an average value of 457 mg C me2 day-’ during Zone NP5 (Fig. 2b). The same dramatic productivity peak was found at all three sites (758, 762, 752) where the record for Zone NP5 could be measured. Productivity then falls precipitously to around the loo-120 mg C mm2 day-’ level during the late Paleocene (NP6-NP7-8). The latest Paleocene-early Oligocene (NP9-NP23) accumulation record is unreliable because of hiatuses and winnowing, and productivity was not calculated

78

W.G. Siesser / Global and Planetary Change I1 (I 995) 71-88

for this interval. Late Oligocene through middle Miocene (NP24-NN8) productivity is low. Productivity begins to rise again during early late Miocene (NN9), reaching a peak of 328 mg C m-* day- ’ during Zone NNlO. Productivity values in the latest Miocene, Pliocene, and Pleistocene (NNl l-NN19) are fairly uniform, ranging between 158 and 202 mg C m- * day - ‘. The calculated Pleistocene value was

158 mg C mm2 day-‘; present-day productivity in this area is in the 96-164 mg C m-* day-’ range (Berger and Wefer, 1991). 5.3. Site 757 This site is on the central portion of Ninetyeast Ridge. The Paleocene-earliest Eocene interval is

700 -

a

650 --

El

600-550 500

2&

,F 400

8350

3

08~

e 250 200 IS0 100 SO 0 19 16 1% 12 11 10 9 S-6 5 Cl 2+ 23 22 21 1% 16 IS 14 13 12 11 10 9 @+7 6 25 m 17 18 15 NP Zmeu INNzonesl

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P/e

e

Plio

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me/

Miocene

e

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e

m

Paleocene

19 16- 1% 12 11 10 9 8-6 5 4-l 24 23 22 21 1s 16- 15 14 13 12 11 10 9 S-7 6 25 M 17 I8 1s

]NPZmes

NNzoawl

I

P/e

e

P/i0

I

Miocene

me/

e

I

Oligo

Fig. 3. Temporal changes in Tertiary productivity. Graphs of mg C m-’ Martini’s calcareous nannofossil zones. a. Site 757. b. Site 762.

Eocene day-’

5 4-3 2

1

e

m

e

I

Eocene

o/igo

5 4-3 2

I

e

Paleocene

(mean values for each zonal interval)

plotted against

W.G. Siesser / Global and Planetary Change 1I (1995) 71-88

composed of volcanic ash. Calculation of the MAR in the first calcareous interval (NP13) shows that high productivity, averaging 355 mg C me2 day-’ occurred during the late early Eocene (Fig. 3a). Productivity dropped sharply to 133 mg C m-2 day-’ in the next zone, then declined gradually but continuously, reaching a low of 29 mg C mm2 day-’

79

during the late Oligocene (NP24-25). Productivity remained low during the early and middle Miocene (NNl - NN8), but increased over its Oligocene low. The late Miocene and early Pliocene intervals (NN9 - NN13-15) have pronounced increases and decreases in productivity. Average productivity appears to be more uniform in the late Pliocene (104 mg C

700 650 600 550 500 FW 3 400 a350 3 om300

1 \

8250 200 1.50

19 1c 1c 12 11 10 9 a-6 s 4-l 24- 23 22 21 lb 1c 15 14 13 12 11 10 9 8-7 6 18 15 25 20 17 Np zone3 (NNzmes~ I

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me/

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O/igo

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600 -550 -so0 -$+450-Ploo 3e 350~-

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12 11 10 9 8.4 5 4-l 24. 23 22 21 l& 16 15 14 13 12 11 10 9 8-7 6

18 15

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Fig. 4. Temporal changes in Tertiary productivity. Graphs of mg C m -’ day-’ Martini’s calcareous nannofossil zones. a. Site 752. b. Site 748.

1 I

e

Paleocene

(mean values for each zonal interval)

plotted against

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m -2 day-‘) and Pleistocene (101 mg C mm2 day- ‘). Productivity in this region today ranges from 41 to 96 mg C m-’ day- 1 (Berger and Wefer, 1991). 5.4. Site 762 Site 762 is on the central part of the Exmouth Plateau. As at Sites 752 and 758, a productivity peak (444 mg C m- 2 day- ‘) occurred during the late Paleocene Zone NP5 (Fig. 3b). Productivity fluctuated in the 140-225 mg C mm2 day-’ range for the rest of the Paleocene and earliest Eocene (NP6NPlO), increased to 290 mg C mm2 day-’ in the middle early Eocene (NPll), but decreased to just below 100 mg C me2 day-’ by the end of the early Eocene. Productivity remained at or just above that level through the rest of the early and middle Eocene (NP12-NP17), then rose sharply in the late Eocene (NP18-20). The record is incomplete for the earliest Oligocene, but the rest of the Oligocene (NP22NP25) experienced very low productivity. Virtually all the Miocene is unmeasurable because of several brief hiatuses. In the early Pliocene (NN12), productivity rose from 80 to 283 mg C rnp2 day-‘, then fell in a regular fashion to 214 mg C me2 day-’ in the late Pliocene (NN16-18) and to 167 mg C me2 day-l during the Pleistocene. Present-day productivity over this site ranges from 164 to 274 mg C mm2 day-’ (Berger and Wefer, 1991). 5.5. Site 752 Site 752 lies near the crest of Broken Ridge. Productivity rose sharply at this site from 104 mg C me2 day-’ in early Paleocene Zone NP2 to a peak of 638 mg C mm2 day-’ during late Paleocene Zone NP5 (Fig. 4a), the highest average productivity measured in this study. As at Sites 758 and 762, where this peak also occurs, productivity then fell sharply in the late Paleocene to the 200 mg C me2 day-’ range (NP6-NP9). Productivity remained low until early Eocene, when productivity increased again to 427 mg C m-2 day-’ in Zone NPll. The stratigraphic record from early Eocene to early Miocene contains hiatuses, as well as sediments that are too poorly preserved to warrant UAR calculations. The record from early middle Miocene through the Pliocene (NN5-NN16-18) is good, however, and

shows that a major reduction in productivity occurred between the Paleogene and Neogene. Values in the Neogene interval are relatively low, all in the 50-100 mg C m-’ day-’ range and show little variation. 5.6. Site 748 Site 748 is a high-latitude site on the southern Kerguelen Plateau. The record at this site is the least complete of the sites selected for study, owing to inadequate preservation as well as inferred winnowing during a number of stratigraphic intervals. Productivity nevertheless shows the same high level during early Eocene Zone NPll that was seen at Sites 752 and 762 (Fig. 4b). Scattered points from Eocene to early Miocene indicate overall decreasing productivity with time; calcareous productivity in the late Pliocene (NN16-18) was a low 20 mg C rn--’ day-‘.

6. Spatial productivity Sites 709, 748, 752, 757, 758, and 762 were located in the relatively small area occupied by the nascent Indian Ocean during the early Paleocene. Productivity averages were consistently low during the earliest Paleocene (Fig. 5a), never exceeding 116 mg C me2 day-‘. A southern belt of low productivity which stretched from near Australia westward to mid ocean has, however, higher productivity than at Site 758, slightly north of the belt. Productivity values increased markedly by late Paleocene (Fig. 5b): the southernmost region of the ocean was producing above 300 mg C m-’ day-‘, and the northernmost sites analyzed produced between 245 and 300 mg C m-2 day-‘. Again, the southern sites show somewhat higher productivity than the northern sites. Productivity during the early Eocene (Fig. 6a) was still high, although not quite as high as during the late Paleocene. Moreover, productivity lacked a clear latitudinal trend. Highest productivity occurred in the middle of the ocean, whereas lower values occurred on the Exmouth and Kerguelen Plateaus. Productivity continued to decrease during the middle Eocene (Fig. 6b), at least at sites where measure-

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Global and Planetary Change I I (1995) 71-88

81

90” \

&

Fig. 5. Spatial changes in Tertiary productivity. Productivity ranges represent mean values for each sub-epoch. a. early Paleocene. b. late Paleocene.

ments could be made. A belt of productivity in the loo-150 mg C mm2 day-’ range stretched from the middle of the ocean across to near Australia, whereas lower productivity occurred on the Madingley Rise to the north, between southern India and Africa. During the late Eocene (Fig. 7a), productivity patterns changed, with high productivity (200-300 mg C me2 day- ‘) occurring on the Madingley Rise and Exmouth Plateau, but low productivity (50-150 mg

-.

1st

“..

Fig. 6. Spatial changes in Tertiary productivity. Productivity ranges represent mean values for each sub-epoch. a. early Eocene. b. middle Eocene.

C mW2day- ‘1 on the southern Ninetyeast Ridge and Kerguelen Plateau. Productivity decreased in a regular pattern from northwest to southeast across the Indian Ocean during the early Oligocene (Fig. 7b). Centrally located Site 757 produced virtually the same amount of carbon as it did in the late Eocene, but to the southeast, average productivity on the Exmouth Plateau decreased from 255 to 46 mg C md2 day-‘.

82

W.G. Siesser / Global and Planetary Change 1 I f 1995) 71-88

Fig. 7. Spatial changes in Tertiary productivity. Productivity ranges represent mean values for each sub-epoch. a. late Eocene. b. early Oligocene.

Fig. 8. Spatial changes in Tertiary productivity. Productivity ranges represent mean values for each sub-epoch. a. late Oligocene. b. early Miocene.

Low productivity (O-50 mg C me2 day-‘) continued through the late Oligocene (Fig. Sa) on the Exmouth Plateau and on the southern part of the Ninetyeast Ridge. On the northern part of the Ninetyeast Ridge, productivity was slightly higher (loo150 mg C me2 day-‘). Low productivity still prevailed in the early Miocene (Fig. 8b), with a belt of productivity in the O-50 mg C m-* day-’ range extending from the

Kerguelen Plateau northward to Site 757 on the Ninetyeast Ridge. Site 758 to the north was somewhat more productive (150-200 mg C m-* day ‘1 at this time. The middle Miocene (Fig. 9a) was the time of the most uniform productivity in the Indian Ocean during the Tertiary. Productivity in the 50-100 mg C m-* day-’ range stretched from Site 758 in the north to Site 752 in the south, and westward to Site 709. The uniform pattern of productivity broke

W.G. Siesser / Global and Planetary Change I I (I 995) 71-88 IO’ I

-3

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Fig. 9. Spatial changes in Tertiary productivity. Productivity ranges represent mean values for each sub-epoch. a. middle Miocene. b. late Miocene.

in the late Miocene (Fig. 9b); high productivity stretched in a belt across the northern Indian Ocean from Site 709 to 758, but decreased progressively to the south. The early and late Pliocene had much the same productivity distribution as the late Miocene, with higher productivity in the north, decreasing progressively to the south (Fig. lOa,b). Productivity at Site 762 off Australia was also relatively high during the up

I.

Pliocene

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Fig. 10. Spatial changes in Tertiary productivity. Productivity ranges represent mean values for each sub-epoch. a. early Pliocene. b. late Pliocene.

Pliocene, and increased slightly from early to late Pliocene (150-200 to 200-250 mg C m-’ day- ‘1.

7. Synoptic history of productivity

in the Indian

Ocean

Productivity was clearly variable in both time and space in the Indian Ocean during the Tertiary Period.

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Primary productivity in the oceans is determined by a complex combination of controls: nutrient supply, solar irradiation, water-mass stability, temperature, oceanic circulation, cloud cover, turbidity, etc. It is unlikely that all the controls on paleoproductivity for a given time interval can ever be inferred. Nevertheless, we can make some guesses as to what may have influenced gross changes in calcareous nannoplankton primary productivity, and the associated secondary productivity by planktic foraminifers. The oceanic area bordered by India, Australia, Antarctica, and Africa (the nascent Indian Ocean) was a much more constricted body of water during the Paleocene to middle Eocene than during the rest of the Tertiary. Currents circulating through this small ocean during the late Paleogene and Neogene were vigorous (see Haq, 1984), in contrast to the relatively sluggish early Paleogene circulation (Zachos et al., 1993); the world’s climate was warm, even at high latitudes; and fluvial input of nutrients from the closely surrounding, high-standing land masses would have been greater at these sites during the Paleogene than during the later Cenozoic when the continents had moved farther apart. The early Tertiary Indian Ocean may well have been almost mesotrophic, rather than truly oligotrophic. Such a warm, narrowly confined sea with an adequate supply of nutrients would have been ideal for blooms of calcareous nannoplankton. The circulation map of Haq (1984) indicates no coastal upwelling (which would have favored siliceous productivity) for this region during the early late Paleocene to middle Oligocene. The sharp increase in productivity among the calcareous nannoplankton and planktic foraminifers during Zone NP5 of the early Paleocene was a major paleoceanographic event, and occurred at all three sites where data are available. There is no evidence of a single cause to explain this productivity peak. However, the findings of Zachos et al. (1993) may provide a clue as to the cause of high productivity at this time. They found that periods of brief, but extreme, climatic change are superimposed on longer term Paleogene climatic trends. Moreover, these abrupt climatic fluctuations were accompanied by reorganization of Paleogene oceanic circulation and by major changes in productivity (Zachos et al., 1993). The peaks during Zone NP5 and Zone NPll

may reflect similarly brief, climate-controlled productivity increases. Barron and Peterson (1991) found that large changes in the circulation of the Indian Ocean occurred after the Eocene, when the first signs of ‘‘modem’ ’ ocean circulation developed. By middle or late Oligocene, surface-water circulation patterns were beginning to look much like modem oceanic patterns (Haq, 19841, and the central area of the ocean was probably truly oligotrophic by this time, if not before. The temporal plots (Figs. 2-4) suggest that productivity generally underwent a sustained fall throughout the ocean beginning in the Oligocene. It is tempting to suggest that this lowered productivity may be related to the marked global cooling which began in early middle Eocene (Aubry, 1992; Zachos et al., 1993). The dramatic late Paleogene cooling initiated a major change from “Greenhouse” earth to the eventual “Icehouse” earth of the Neogene and Quaternary. The Antarctic ice cap began to grow, steeper latitudinal thermal gradients began to develop, bottom-water temperatures in the oceans dropped 7-8” C (Zachos et al., 1993) initiating the development of the psychrosphere, and circulation in the world’s oceans changed from predominantly thermospheric to thermohaline (Haq, 1984). Cooler surface waters and stronger global wind systems developed as a result of global cooling, and intensified oceanic circulation produced greater mixing and upwelling, thereby bringing more nutrients into the euphotic zone. Siliceous primary producers seem to have been the main beneficiaries wherever late Tertiary upwelling occurred, with extensive deposits of diatomaceous sediments forming (Diester-Haass, 1978; Stein, 1985; Barron and Baldauf, 1989). Diatoms flourish and dominate in nutrient-rich upwelling zones, whereas calcareous nannoplankton dominate among the phytoplankton where nutrient supplies are scarce (Hallock et al., 1991). Even within some upwelling zones, diatoms and coccolithophores may show different patterns of abundance. During pulses of newly upwelled, high-nutrient water, diatoms will dominate, whereas coccolithophores dominate in more mature, nutrient-depleted upwelled water (Mitchell-Innes and Winter, 1987; Giraudeau et al., 1993). Nutrient uptake kinetics may explain part of this pattern of diatom and calcareous nannoplankton

W.G. Siesser / Global and Planetary Change I1 (1995) 71-88

distribution. Giraudeau et al. (1993) noted that coccolithophores have a much lower K, value than diatoms (K, = the minimum nutrient concentration at which a given species can grow). This would presumably limit or preclude the growth of diatoms, but not calcareous nannoplankton, in nutrient-deficient waters. Phytoplankton in the vast, central, low- and midlatitude oceanic areas generally benefitted less from the intensified wind and current circulation than coastal phytoplankton. These are areas mostly enclosed by major ocean gyres and are biologically impoverished because nutrient supplies are low. Establishment of modem oceanic and atmospheric circulation during the Neogene (Haq, 1984; Barron and Peterson, 1991) consigned most of the central oceanic areas to long-term oligotrophic conditions, where calcareous namroplankton are the major primary producers. Figs. 9b-lob show greater N-S differences in productivity in the late Neogene than in earlier times. This is possibly because of the larger latitudinal ocean expanse by this time, and continued cooling of the southern high latitudes. Calcareous productivity remained at low levels at the southern sites during the late Miocene and Pliocene, but increased at the northern sites at the same time before dropping slightly during the late Pliocene and Pleistocene. Climatic instability, as described by Zachos et al. (1993) for the Paleogene, with abrupt, short-term climatic changes superimposed on the longer-term trend, may explain the marked fluctuations in productivity during the late Neogene. The development of extremely cold waters at high latitudes probably accounts for the relative decrease in high-latitude calcareous productivity during the late Neogene.

8. Summary Primary productivity in the oceans is an important process with ramifications for biological, geological, and chemical oceanography. Productivity removes enormous amounts of carbon and smaller amounts of other elements from ocean waters. Vast quantities of these elements eventually become locked away in calcareous and siliceous sediments on the sea floor and in onshore deposits that originally formed in

85

ancient oceans. The geologic record of the carbon system is thus important in understanding global change. Paleoproductivity in the oligotrophic areas of the world’s oceans has been investigated less than in the more fertile nearshore areas, in part because of the absence of a reliable proxy to measure productivity in oligotrophic areas. Brummer and Van Eijden (1992) have recently provided a transfer function based on carbonate AfAR, and their equations have been used in this study to investigate paleoproductivity of the oligotrophic Indian Ocean during the Tertiary. Calculations show that productivity fluctuated on both a spatial and temporal basis during the Tertiary, primarily reflecting changes in nutrient supply, climate, and oceanic circulation. Early Paleogene productivity in the Indian Ocean was considerably higher than in the late Paleogene and Neogene. Productivity increased sharply during early Paleocene nannofossil Zone NP5 and during Eocene Zone NPll. These productivity peaks may be related to abrupt changes in climate which caused reorganization of oceanic circulation. Global cooling and the initiation of the modem Indian Ocean circulation system caused productivity to decrease in the Oligocene. This decrease in productivity continued into the Neogene. Generally low productivity characterized the Neogene, although periods of higher productivity occurred, especially at the more northern sites in the Indian Ocean.

Acknowledgements I thank Jennifer Shepard for helping with preparation of the graphs and maps, and for reading an early draft of the manuscript. Dr. Ton van Eijden also read a draft of the manuscript and made many helpful comments.

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