Benthic Foraminiferal δ18O Research Articles

Late Pleistocene-Holocene Paleoceanography and Ventilation of the Gulf of California

Sediment cores collected in 1990 from the Gulf of California have been studied using stable isotope and radiocarbon techniques to reconstruct the climate and ventilation histories since the last glacial maximum. Benthic foraminiferal δ18O from core tops in a water depth range of 145 to 1442 m increases by about 2% with increasing depth. This is consistent with a composite temperature profile constructed from several hydrocasts in the various gulf basins. However, the δ18O water/salinity relationship is not sufficiently linear in gulf locations or in nearby open Pacific Geochemical Ocean Sections Study (GEOSECS) stations to be useful in solving paleotemperature equations. Of the most common benthic foraminifera, only Planulina ariminensis has δ13C that is consistent with the measured δ13C of ΣCO2. Several cores in the depth range 500 to 900 m have the laminated Holocene and Bolling/Allerod sediments, and the nonlaminated glacial age and Younger Dryas sediments that are typical of the gulf and other locations such as Santa Barbara Basin. The best of those, Jumbo Piston Core (JPC) 56 from 818 m water depth on the western margin of Guaymas Basin, was sampled for intensive study. Oxygen isotope ratios in benthic and planktonic foraminifera show little evidence for deglacial temperature oscillations. Carbon isotope ratios are generally lower during warm epochs, but the most striking result is strongly lowered benthic and planktonic δ13C about 9500 years ago. This may reflect water column oxidation of locally released methane. Neither benthic δ13C in depth section nor paired benthic and planktonic 14C data in JPC56 are consistent with increased intermediate water ventilation during the glacial maximum and Younger Dryas. Likewise, 14C data from 5 pairs of foraminifera from the Okhotsk Sea fail to support better ventilation in that basin during the last glacial maximum.

Last glacial benthic foraminiferal δ18O anomalies in the polar North Atlantic: A modern analogue evaluation

Modern processes are evaluated to understand the possible mechanisms behind last glacial benthic foraminiferal δ18O anomalies that occurred concurrent with meltwater events in the polar North Atlantic; such anomalies in the Nordic seas were recently interpreted to be caused by brine formation. Despite intensive sea‐ice production on circumarctic shelves, modern data show that brines ejected from sea‐ice formation containing low δ18O water do not significantly contribute to deep waters in the Arctic Ocean today. Assuming that this process was, nevertheless, responsible for δ18O anomalies in Nordic seas deep water during the last glaciation, a broad, shallow shelf area adjacent to the Nordic seas, such as the Barents Sea, had to be seasonally free of sea‐ice in order to serve as an area for brine formation. Another process which may explain δ18O‐depleted water at depth is found in the Weddell Sea today, where a low δ18O signal in deep waters originates from ice shelf interactions. If temperature were considered the main mechanism for the low benthic δ18O values, an increase of 4°C must have occurred in the deep water. An analogous situation with a reversed water temperature pattern due to a subsurface inflow of warm Atlantic water is found today in the eastern Arctic Ocean, and deep water warming is observed in the Greenland Gyre in the absence of deep convection. Because paleoproxy data also indicate an Atlantic water inflow into the Nordic seas during such benthic δ18O anomalies, temperature as a principal mechanism of changing δ18O cannot be excluded.

Pore fluid constraints on deep ocean temperature and salinity during the Last Glacial Maximum

Pore water records of δ18O and [Cl] from ODP Site 1063A on the Bermuda Rise constrain the change in seawater δ18O and salinity from the Last Glacial Maximum (LGM) to the Holocene to be 0.75±0.05‰ and 2.5±0.1% respectively. Coupled with a measured benthic foraminiferal δ18O change, this result means that bottom waters were 4.6±0.8°C cooler than the Holocene at the LGM and therefore at or near the seawater freezing point. Coupled δ18O and chlorinity results give an extrapolated mean ocean LGM to Holocene change in δ18O of 0.95±0.09‰. These data also constrain the past southern source deep‐water salinity to be 35.76±0.04 psu, which is within error of the mean deep ocean value for this time.

North Atlantic Intermediate to Deep Water circulation and chemical stratification during the past 1 Myr

Benthic foraminiferal carbon isotope records from a suite of drill sites in the North Atlantic are used to trace variations in the relative strengths of Lower North Atlantic Deep Water (LNADW), Upper North Atlantic Deep Water (UNADW), and Southern Ocean Water (SOW) over the past 1 Myr. During glacial intervals, significant increases in intermediate‐to‐deep δ13C gradients (commonly reaching >1.2‰) are consistent with changes in deep water circulation and associated chemical stratification. Bathymetric δ13C gradients covary with benthic foraminiferal δ18O and covary inversely with Vostok CO2, in agreement with chemical stratification as a driver of atmospheric CO2 changes. Three deep circulation indices based on δ13C show a phasing similar to North Atlantic sea surface temperatures, consistent with a Northern Hemisphere control of NADW/SOW variations. However, lags in the precession band indicate that factors other than deep water circulation control ice volume variations at least in this band.

Surface and deep water changes in the subpolar North Atlantic during Termination II and the Last Interglaciation

To reconstruct the history of water mass exchange between the NE Atlantic and the Nordic seas, sediment cores from ∼2 km water depth were studied across Termination II (TII) and through the last interglaciation (MIS5e). During early TII the sudden appearance of the low‐latitude planktonic foraminifera Beella megastoma is noted in both regions along with a steep decrease in benthic foraminiferal δ18O. Since other proxies indicate that surface waters were cold and stratified because of meltwater, conditions which prevented near‐surface thermohaline circulation and vertical convection in the Nordic seas, water mass exchange between the two areas occurred at the subsurface. During later TII, surface conditions changed, and this subsurface circulation style was eventually replaced by vertical convection. In the Nordic seas, B. megastoma vanished from the record together with ice‐rafted debris (IRD) at the end of TII, while subpolar foraminiferal abundance rose. Peak interglacial conditions with intensive vertical convection now fully developed, generating a bottom water temperature gradient of ∼4°C between the two areas. However, surface water temperatures deteriorated in the Nordic seas already notably before IRD recurred, and δ18O increased at the end of MIS5e.

Changes in Meridional Temperature and Salinity Gradients in the North Atlantic Ocean (30°–72°N) during the Last Interglacial Period

Eight deep‐sea sediment cores from the North Atlantic Ocean ranging from 31° to 72°N are studied to reconstruct the meridional gradients in surface hydrographic conditions during the interval of minimum ice volume within the last interglacial period. Using benthic foraminiferal δ18O measurements and estimates of Sea Surface Temperature (SST) and Sea Surface Salinity (SSS), we show that summer SSTs and SSSs decreased gradually during the interval of minimum ice volume at high‐latitude sites (52°–72°N) whereas they were stable or increased during the same time period at low‐latitude sites (31°–41°N). This increase in meridional gradients of SSTs and SSSs may have been due to changes in the latitudinal distribution of summer and annual‐average insolation and associated oceanic and atmospheric feedbacks. These trends documented for the Eemian ice volume minimum period are similar to corresponding changes observed during the Holocene and may have had a similar origin.

Glacial‐age hydrography of the far northwest Pacific Ocean

Stable isotope data on benthic foraminifera from more than 30 cores on the northern Emperor Seamounts and in the Okhotsk Sea are synthesized in paleohydrographic profiles spanning the depth range 1000–4000 m. Holocene (core‐top) benthic foraminiferal δ18O and δ13C data are calibrated to modern hydrographic properties through measurements of δ13C of ΣCO2 and δ18O of seawater. Cibicidoides stable isotope ratios are close to the δ13C and equilibrium δ18O of seawater, whereas Uvigerina δ18O and δ13C are variably offset from Cibicidoides. Glacial maximum δ13C of Cibicidoides displays a different vertical profile than that of the Holocene. When results are adjusted by +0.32‰ to account for the secular change in δ13C during the last glacial maximum, the data coincide with the modern seawater and foraminiferal curves deeper than ∼2 km. However, at shallower depths δ13C gradually increases by as much as 1‰ above the modern value. Furthermore, above 2 km the benthic δ18O decreases by ∼0.5‰. These results are consistent with a benthic front at ∼2 km in the North Pacific [see Herguera et al., 1992], but they differ from interpretations based on trace metal data which indicate a source of nutrient‐depleted deep water during glaciation. The isotopic data suggest that during glaciation there was a better ventilated watermass at intermediate depths in the far northwestern Pacific, it was relatively fresher than deep waters there, and deep waters were as nutrient‐rich as today.

Precise dating of the Holmatindur cooling event in eastern Iceland: Evidence for mid‐Miocene bipolar glaciation

A succession of basaltic lavas, volcaniclastic sediments, and lignite beds in the Holmatindur region of eastern Iceland provides the means for tying magnetic reversal stratigraphy and a record of major North Atlantic cooling to a precise radiometric timescale. A prominent part of the section is the Holmatindur clastic bed, which is up to 66 m thick and can be traced 80 km along strike. The geological significance of this unit lies in its composition and associated thin lignite seams. The volcaniclastic composition is basaltic (in contrast to the common rhyolitic/dacitic composition of tuff units in the area), and hyaloclastic material is dominant. Hydroexplosive volcanism most likely produced the hyaloclastite, through subglacial eruptions within the paleovolcanic zone. Pollen assemblages from the lignite beds indicate a dramatic climatic deterioration, from subtropical to cool temperate conditions, through these sediments. Feldspar crystals separated from the Holmatindur clastic bed were determined to be from 10.72 (± 0.16) Ma, from 40Ar‐39Ar incremental heating experiments. We correlate the Holmatindur cooling event with a relative maximum in benthic foraminiferal δ18O (Zone Mi6 of Miller et al. [1991]), which has been interpreted as an episode of ice accumulation within the mid‐Miocene cooling period, as well as with pulses in ice‐rafted debris supplied to deep water sites in the North Atlantic Ocean, a global sea level drop of 70 m, and submarine canyon cutting, all of which occurred within the early part of magnetic anomaly 5 normal (C5n). While strong evidence exists for mid‐Miocene ice sheets in Antarctica, northern hemisphere glaciation is thought to have started much later. The Iceland sites studied here demonstrate that significant ice accumulation occurred, albeit intermittently, as early as late middle Miocene time in the North Atlantic Ocean. From radiometric dating of Tertiary lavas that lie below and above the well‐established lower boundary C5n, we conclude that the onset of this long normal polarity period occurred at 10.94 (± 0.16) Ma. The new data are compatible with the Cande and Kent [1995] interpolated estimate for this boundary (10.95 Ma) but are significantly older, or more precise, than previous direct dating by conventional K‐Ar methods.

Paleocene to Early Eocene paleoceanography of the Middle East: The δ13C and δ18O isotopes from foraminiferal calcite

Paleocene to early Eocene benthic foraminiferal δ13C and δ18O records from southern Tethyan sections at Ben Gurion, Israel (paleodepth 500–700 m), and Gebel Aweina, Egypt (paleodepth 150–200 m), show generally similar trends but 1–3‰ more negative values than coeval deep‐sea isotopic records. In both Tethyan sections a negative δ13C excursion of 2.5–3‰ marks the benthic extinction event in the latest Paleocene. For at least 1 m.y. after this event, δ13C values were 1.5–2‰ more negative on the shelf than at upper bathyal depths, reflecting a deepening of the oxygen minimum zone, possibly related to an increase or spatial shift in upwelling. Benthic δ18O records indicate a 2–4°C temperature gradient between the shelf and upper bathyal depths. Temperature‐salinity reconstructions suggest that upwelling was a dominant mechanism for surface water formation in this part of the southern Tethys during the late Paleocene.

Millennial‐scale deep water oscillations: Reflections of the North Atlantic in the deep Pacific from 10 to 60 ka

Northeast Pacific benthic foraminiferal δ18O and δ13 reveal repeated millennial‐scale events of strong deep‐sea ventilation (associated with nutrient depletion and/or high gas exchange) during stadial (cool, high ice volume) episodes from 10 to 60 ka, opposite the pattern in the deep North Atlantic. Two climate mechanisms may explain this pattern. North Pacific surface waters, chilled by atmospheric transmission from a cold North Atlantic and made saltier by reduced freshwater vapor transports, could have ventilated the deep Pacific from above. Alternatively, faster turnover of Pacific bottom and mid‐depth waters, driven by Southern Ocean winds, may have compensated for suppressed North Atlantic Deep Water production during stadial intervals. During the Younger Dryas event (∼11.6–13.0 cal ka), ventilation of the deep NE Pacific (∼2700 m) lagged that in the Santa Barbara Basin (∼450 m) by >500 years, suggesting that the NE Pacific was first ventilated at intermediate depth from above and then at greater depth from below. This apparent lag may reflect the adjustment time of global thermohaline circulation.

High‐resolution (104 years) deep‐sea foraminiferal stable isotope records of the Eocene‐Oligocene climate transition

We have constructed high‐resolution (104– 105 years) benthic foraminiferal δ13C and δ18O records for the upper Eocene through lower Oligocene of two pelagic sequences, Deep Sea Drilling Project (DSDP) Site 522 in the Angola Basin, South Atlantic Ocean, and Ocean Drilling Program (ODP) Site 744 in the southern Indian Ocean. These records provide improved constraints on both the timing and magnitude of marine oxygen and carbon isotope events from 30 to 35 Ma. The oxygen isotope records indicate that the ubiquitous δ18O increase (Oi‐1), which marks the rapid expansion of continental ice sheets and a minimum of 3° to 4°C of cooling of bottom waters in the earliest Oligocene (33.6 Ma), occurred in <350 kyr. More than half the transition occurred over the final 40–50 kyr. This period of lower temperatures and widespread continental glaciation persisted for roughly 400 kyr (i.e., the duration of magnetochron C13n). These records also indicate that this interval was characterized by at least two ∼ 100‐kyr waxing and waning cycles (Oi‐1a and Oi‐1b) and possibly several higher‐frequency events. The benthic foraminiferal δ13C records show a positive 0.8‰ excursion that is nearly isochronous with the Oi‐1 oxygen isotope increase. Similar magnitude δ13C increases at other sites indicate this was a global phenomenon suggestive of an unusually large perturbation to the carbon cycle. This excursion was followed by smaller amplitude δ13C oscillations with periods of roughly ∼400 kyr. We suspect that the ubiquitous Oi‐1 δ13C excursion resulted from a brief but substantial increase in export production and carbon burial.

An Integrated Latest Quaternary (Stage 3 to Present) Paleoclimatic and Paleoceanographic Record from Offshore Northern New Zealand

Paleoceanographic and onshore paleoclimatic changes during the last 59,000 yr are established from three deep-sea sediment cores off northeast New Zealand using an integrated log of sediment texture, CaCO3 content, palynology, and planktonic and benthic foraminiferal δ18O and δ13C data, together with dated silicic tephras. These records from the isotopic stage 4-3 boundary to the present record northern New Zealand vegetation history, changes in a subsidiary equatorward flow of Circumpolar Deep Water, and sea-surface temperatures (SSTs) for subtropical water (STW) between latitudes 36°42′ and 35°51′S. Relative to the Holocene, isotopically derived SSTs record average changes of +2°C, -2°C, and -2°C for the 59,000-43,000, 43,000-24,000, and 24,000-12,000 yr time slices, respectively. The apparent +2°C warming for the 59,000-43,000 yr period is interpreted to reflect changes in the dominant depth habitat of Globigerina bulloides in response to upwelling. A -2°C cooling of SSTs during isotope stage 2 is interpreted, in part, to reflect upwelling of cool subsurface water resulting from strong and persistent westerly airflow across New Zealand, with the concomitant enhanced surface-water production of CaCO3. Onshore, vegetation consistent with these changes are recorded, with full conifer-hardwood forest prior to 43,000 yr, followed by a change to vegetation implying cooler and drier conditions between 43,000 and 12,000 yr, and a subsequent return to full forest during the Holocene. The sequence of biopelagic and hemipelagic sedimentation observed within these cores reflect oscillation of sea level about a threshold eustatic level that controls the transport of terrigenous detritus offshore. Local variations and interplay of the regional oceanography and morphology and tectonism of the continental shelf will dictate that, relative to present sea level, this threshold eustatic sea level will vary in depth, and hence age, along a continental margin. Data from the New Zealand region reveal an extremely steep meridional thermal gradient across the southern and central New Zealand region during the last glaciation with minor cooling of STW to the north, apart from localized nearshore upwelling zones, but pronounced cooling of subantarctic water to the south of the subtropical convergence zone.

Late Paleocene to Eocene paleoceanography of the equatorial Pacific Ocean: Stable isotopes recorded at Ocean Drilling Program Site 865, Allison Guyot

An expanded and largely complete upper Paleocene to upper Eocene section was recovered from the pelagic cap overlying Allison Guyot, Mid‐Pacific Mountains at Ocean Drilling Program (ODP) Site 865 (18°26′N, 179°33′W; paleodepth 1300–1500 m). Reconstructions show that the site was within a few degrees of the equator during the Paleogene. Because no other Paleogene sections have been recovered in the Pacific Ocean at such a low latitude, Site 865 provides a unique record of equatorial Pacific paleoceanography. Detailed stable isotopic investigations were conducted on three planktonic foraminiferal taxa (species of Acarinina, Morozovella, and Subbotina). We studied benthic foraminiferal isotopes at much lower resolution on species of Cibicidoides and Lenticulina, Nuttallides truempyi and Gavelinella beccariiformis, because of their exceptional rarity. The δ18O and δ13C stratigraphies from Site 865 are generally similar to those derived from other Paleocene and Eocene sections. The planktonic foraminiferal records at Site 865, however, include significantly less short‐term, single‐sample variability than those from higher‐latitude sites, indicating that this tropical, oligotrophic location had a comparatively stable water column structure with a deep mixed layer and less seasonal variability. Low‐amplitude (0.1–0.8‰) oscillations on timescales of 250,000 to 300,000 years correlate between the δ13C records of all planktonic taxa and may represent fluctuations in the mixing intensity of surface waters. Peak sea surface temperatures of 24°–25°C occurred in the earliest Eocene, followed by a rapid cooling of 3–6°C in the late early Eocene. Temperatures remained cool and stable through the middle Eocene. In the late Eocene, surface water temperatures decreased further. Vertical temperature gradients decreased dramatically in the late Paleocene and were relatively constant through much of the Eocene but increased markedly in the late Eocene. Intermediate waters warmed through the late Paleocene, reaching a maximum temperature of 10°C in the early Eocene. Cooling in the middle and late Eocene paralleled that of surface waters, with latest Eocene temperatures below 5°C. Extinction patterns of benthic foraminifera in the latest Paleocene were similar to those observed at other Pacific sites and were coeval with a short‐term, very rapid negative excursion in δ13C values in planktonic and benthic taxa as at other sites. During this excursion, benthic foraminiferal δ18O values decreased markedly, indicating warming of 4 to 6°C for tropical intermediate waters, while planktonic taxa show slight warming (1°C) followed by 2°C of cooling. Convergence of δ18O values of planktonic and benthic foraminifera suggests that thermal gradients in the water column in this tropical location collapsed during the excursion. These data are consistent with the hypothesis that equatorial Pacific surface waters were a potential source of warm, higher salinity waters which filled portions of the deep ocean in the latest Paleocene. Oxygen isotopic data indicate that equator to high southern latitude sea surface thermal gradients decreased to as little as 4°C at the peak of the excursion, suggesting some fundamental change in global heat transport.

Magnetostratigraphic, biostratigraphic, and stable isotope stratigraphy of an Upper Miocene drill core from the Salé Briqueterie (northwestern Morocco): A high‐resolution chronology for the Messinian stage

We report a high‐resolution stable isotope, carbonate, magnetostratigraphic, and biostratigraphic record from a 175‐m drill core from the Salé Briqueterie, which is part of the Bou Regreg section in northwestern Morocco. The Salé drill core spans the interval from paleomagnetic Chron C4n partim to C3r (earliest Gilbert), which represents the time leading up to and including the isolation and desiccation of the Mediterranean (i.e., the Messinian salinity crisis). During Chrons C3An and C3Ar (6.935 to 5.894 Ma) the isotope and carbonate signals display quasi‐periodic variations with estimated periods of 40 and 100 kyr, respectively. We interpret the 40‐kyr δ18O variations as reflecting changes in global ice volume caused by obliquity‐induced changes (41 kyr) in solar insolation in polar regions. The 100‐kyr carbonate variations probably represent long‐term modulation of the amplitude of the precessional cycle (∼21 kyr), which is not resolved by our sampling frequency. The cyclic nature of the oxygen isotope signal permits us to extend the isotope nomenclature of Shackleton et al. (1994a) from stage TG24 in Chron C3r (earliest Gilbert) to stage C3Ar.δ18O.18 at the base of Chron C3Ar (6.935 Ma). A major change in paleoceanographic conditions is recorded across the Tortonian/Messinian boundary, which we correlate to Chron C3Bn at 7.04 Ma. Benthic foraminiferal δ18O values increased by an average of 0.4‰ in two steps at 7.17 Ma and 6.8 Ma and δ13C values decreased by 0.7–0.8‰ between 7.1 and 6.8 Ma, representing the late Miocene carbon shift. The first step in δ18O values coincides with an inferred reversal in deep water circulation through the Rifian Corridor, and the second correlates with the base of the Tripoli Formation and onset of “crisis conditions” in the Mediterranean. We suggest that the increase in δ18O values represents, at least in part, an increase in global ice volume that lowered sea level and contributed to the establishment of a negative water budget in the Mediterranean. Average δ18O values remained high throughout most of Chrons C3Ar and C3An, reaching maximum δ18O values during isotope stages TG20 and 22 in Chron C3r (earliest Gilbert). The glacio‐eustatic falls associated with these events may have resulted in the complete isolation of the Mediterranean from the world ocean (Shackleton et al., 1994a). Following stage TG12 in the Salé record, there exists a trend toward progressively lower δ18O values that may represent a series of marine transgressions that eventually reflooded the Mediterranean and ended the Salinity Crisis.

Stratophenetic tracing of phylogeny using SIMCA pattern recognition technique: a case study of the late Neogene planktic foraminiferaGloboconellaclade

The Plio-Pleistocene planktic foraminiferal sequence of theGloborotalia(Globoconella)puncticulata-inflataclade in Deep Sea Drilling Project Site 588, dated at 4.36 Ma to 0.05 Ma, records the branching history of theG. inflatalineage from the ancestralG. puncticulatalineage. The gradational nature of the divergence and the enormous morphological variability inherent in theG. inflatalineage have elicited different views on taxonomy and phylogeny of this clade. A pattern recognition technique, soft independent modeling of class analog (SIMCA), was used as an objective quantitative stratophenetic methodology to reconstruct the phylogenetic history.Typical specimens of two species,G. puncticulataandG. inflata,were identified from a stratigraphic level dated at 2.76 Ma. Principal component models were built to characterize the morphometric patterns of the two morphotypes using SIMCA. TheGloboconellaspecimens of the next lower and higher adjacent stratigraphic levels were evaluated against the models and classified into one of the two morphotypes. The newly classified specimens were then used to build new models for further tracing of lineages in lower and upper sections, respectively. Progression of such training and classification procedures through stratigraphic intervals resulted in a reconstruction of the evolutionary patterns of the two lineages. Cladogenesis gave rise to the descendant lineage,G. inflata,at about 3.5 Ma. The two co-existing species,G. inflataandG. puncticulata,differ only in size and show similarity in most characters at the beginning of their divergence. Other characters began to diverge later, at various rates. The gradients between planktic and benthic foraminiferal δ18O values show a continuous increase during the late Pliocene. The succession fromG. puncticulatatoG. inflataduring the same time correlates with the progressively increased vertical stratification in temperature of surface waters.Globorotalia puncticulatabecame extinct at 2.35 Ma when the temperature gradient further increased, corresponding to the onset of extensive glaciation in the Northern Hemisphere.

Middle Miocene ocean‐climate transition: High‐resolution oxygen and carbon isotopic records from Deep Sea Drilling Project Site 588A, southwest Pacific

High‐resolution stable isotopic records are presented for the epi‐benthic foraminifer Cibicidoides, the inferred shallow‐dwelling planktonic Globigerinoides quadrilobatus, and the inferred deep‐dwelling planktonic Globoquadrina dehiscens from the middle Miocene (∼16–12 Ma) of Deep Sea Drilling Project site 588A, Lord Howe Rise, southwest Pacific. High‐resolution, multiple species oxygen and carbon isotopic data define the timing and character of the well‐known middle Miocene climatic‐oceanographic transition with a resolution comparable to Quaternary records. The benthic foraminiferal δ18O record is marked by several large fluctuations from ∼16 to 14.8 Ma, followed by a series of rapid (<50 kyr) δ18O increases that suggest a new state of the ocean‐climate system after 14.8 Ma. The total middle Miocene benthic oxygen isotopic increase of 1.2‰ is largely incorporated in two steps, an increase of 0.8‰ from 14.5 to 14.0 Ma and a second increase of 0.7‰ from 13.45 to 12.45 Ma. Each step is comprised of a series of marked δ18O increases, indicative of rapid East Antarctic ice sheet growth and contemporaneous deepwater cooling. A strong covariance of 0.7‰ between the benthic and deep‐dwelling planktonic species from 14.5 to 14.0 Ma (including a rapid increase from 14.1 to 14.05 Ma) suggests a 0.7‰ increase in the δ18O composition of seawater (δ18Osw) because of East Antarctic ice sheet growth. Comparison of the δ18O record of Gs. quadrilobatus suggests that surface waters warmed at this site by ∼3°C from 14.1 to 13.6 Ma. Carbon isotopic time series for each species generally covary throughout the early to middle Miocene interval (∼16–12 Ma), confirming that δ13C variations in this interval largely represent reservoir changes. High‐resolution δ13C data allow improved resolution of the latter five of six δ13C maxima within the well‐known early to middle Miocene carbon isotopic excursion (the Monterey Carbon Isotopic Excursion from 17.0 to 13.5 Ma). This is useful for global correlation. The last of these maxima ends with a 1‰ decrease centered from 13.9 to 13.7 Ma, ∼300 kyr after the δ18O increase considered to reflect East Antarctic ice growth. Covariance between benthic δ18O and δ13C from ∼16 to 13.8 Ma suggests a sensitive relation between global carbon cycling and the ocean‐climate system prior to 13.8 Ma. Episodic increases in organic carbon burial may have contributed to deep‐sea benthic δ13C maxima and synchronous global cooling. The positive relationship ended at ∼13.8 Ma, indicative of changing relations between global carbon cycling and the ocean‐climate system brought on by the increased stability of the East Antarctic ice sheet after a major growth phase from 14.5 to 14.0 Ma.

Paleocene to Eocene benthic foraminiferal isotopes and assemblages: Implications for deepwater circulation

Early Paleogene warm climates may have been linked to different modes and sources of deepwater formation. Warm polar temperatures of the Paleocene and Eocene may have resulted from either increased atmospheric trace gases or increased heat transport through deep and intermediate waters. The possibility of increasing ocean heat transport through the production of warm saline deep waters (WSDW) in the Tethyan region has generated considerable interest. In addition, General Circulation Model results indicate that deepwater source regions may be highly sensitive to changing basin configurations. To decipher deepwater changes, we examined detailed benthic foraminiferal faunal and isotopic records of the late Paleocene through the early Eocene (∼60 to 50 Ma) from two critical regions: the North Atlantic (Bay of Biscay Site 401) and the Pacific (Shatsky Rise Site 577). These records are compared with published data from the Southern Ocean (Maud Rise Site 690, Islas Orcadas Rise Site 702). During the late Paleocene, similar benthic foraminiferal δ18O values were recorded at all four sites. This indicates uniform deepwater temperatures, consistent with a single source of deep water. The highest δ13C values were recorded in the Southern Ocean and were 0.5‰ more positive than those of the Pacific. We infer that the Southern Ocean was proximal to a source of nutrient‐depleted deep water during the late Paleocene. Upper Paleocene Reflector Ab was cut on the western Bermuda Rise by cyclonically circulating bottom water, also suggesting a vigorous source of bottom water in the Southern Ocean. A dramatic negative excursion in both carbon and oxygen isotopes occurred in the latest Paleocene in the Southern Ocean. This is a short‐term (<100 kyr), globally synchronous event which also is apparent in both the Atlantic and Pacific records as a carbon isotopic excursion of approximately 1‰. Faunal analyses from the North Atlantic and Pacific sites indicate that the largest benthic foraminiferal faunal turnover of the Cenozoic was synchronous with the isotopic excursion, lending support to the hypothesis that the extinctions were caused by a change in deepwater circulation. We speculate that the Southern Ocean deepwater source was reduced or eliminated at the time of the excursion. During the early Eocene, Southern Ocean δ13C values remained enriched relative to the North Atlantic and Pacific. However, the Southern Ocean was also enriched in δ18O relative to these basins. We interpret that these patterns indicate that although the Southern Ocean was proximal to a source of cool, nutrient‐depleted water, the intermediate to upper deep water sites of the North Atlantic and Pacific were ventilated by a different source that probably originated in low latitudes, i.e., WSDW.

Early Oligocene ice-sheet expansion on Antarctica: Stable isotope and sedimentological evidence from Kerguelen Plateau, southern Indian Ocean

Sedimentological and stable isotope data from a pelagic sequence recovered from the southern Indian Ocean provide the most convincing evidence to date for short-term expansion of a large ice sheet on Antarctica during the earliest Oligocene (∼36 Ma). Terrigenous debris identified as ice-rafted in origin on the basis of textural, compositional, and size criteria is present at the same stratigraphic level as the ubiquitous early Oligocene oxygen isotope shift. The highest benthic foraminiferal δ18O values (>3.0‰) of the Paleogene occur in samples from within the ice-rafted debris interval. These values are similar to those recorded by Holocene benthic foraminifera, implying that the ice sheet may have attained a volume similar to that of the present-day ice sheet on Antarctica. The stratigraphic distribution of ice-rafted debris and high oxygen isotope values indicate that these conditions persisted for roughly 100 ka.

Unlocking the Ice House: Oligocene‐Miocene oxygen isotopes, eustasy, and margin erosion

Oxygen isotope records and glaciomarine sediments indicate at least an intermittent presence of large continental ice sheets on Antarctica since the earliest Oligocene (circa 35 Ma). The growth and decay of ice sheets during the Oligocene to modern “ice house world” caused glacioeustatic sea level changes. The early Eocene was an ice‐free “greenhouse world,” but it is not clear if ice sheets existed during the middle to late Eocene “doubt house world.” Benthic foraminiferal δ18O records place limits on the history of glaciation, suggesting the presence of ice sheets at least intermittently since the earliest Oligocene. The best indicator of ice growth is a coeval increase in global benthic and western equatorial planktonic δ18O records. Although planktonic isotope records from the western equatorial regions are limited, subtropical planktonic foraminifera may also record such ice volume changes. It is difficult to apply these established principles to the Cenozoic δ18O record because of the lack of adequate data and problems in stratigraphic correlations that obscure isotope events. We improved Oligocene to Miocene correlations of δ18O records and erected eight oxygen isotope zones (Oi1‐Oi2, Mi1‐Mi6). Benthic foraminiferal δ18O increases which are associated with the bases of Zones Oil (circa 35.8 Ma), Oi2 (circa 32.5 Ma), and Mil (circa 23.5 Ma) can be linked with δ18O increases in subtropical planktonic foraminifera and with intervals of glacial sedimentation on or near Antarctica. Our new correlations of middle Miocene benthic and western equatorial planktonic δ18O records show remarkable agreement in timing and amplitude. We interpret benthic‐planktonic covariance to reflect substantial ice volume increases near the bases of Zones Mi2 (circa 16.1 Ma), Mi3 (circa 13.6 Ma), and possibly Mi5 (circa 11.3 Ma). Possible glacioeustatic lowerings are associated with the δ18O increases which culminated with the bases of Zone Mi4 (circa 12.6 Ma) and Mi6 (circa 9.6 Ma), although low‐latitude planktonic δ18O records are required to test this. These inferred glacioeustatic lowerings can be linked to seismic and rock disconformities. For example, we link 12 Oligocene‐early late Miocene inferred glacioeustatic lowerings with 12 of the sequence boundaries (= inferred eustatic lowerings) of Haq et al. (1987).

Tertiary ice sheet dynamics: The Snow Gun Hypothesis

We observe strong negative correlation between Tertiary low‐ to mid‐latitude planktonic foraminiferal δ18O and the difference between these data and coeval benthic foraminiferal δ18O. Late Quaternary data do not show this correlation. Coupling statistical model/δ18O comparisons and evidence for Antarctic ice and ocean temperature variation, we infer that Tertiary ice volume, recorded by tropical planktonic δ18O, increased as the deep ocean warmed. Because the isotopic signatures of deepwater temperature variation and ice volume change were of opposite sign, the sum of these signals in Tertiary benthic δ18O became lost in the noise. This renders low correlation between Tertiary planktonic and benthic δ18O time series compared to late Quaternary data. We contend that Tertiary ice sheet growth was commonly driven by warming of deep water from low‐ to mid‐latitude marginal seas (snow gun hypothesis). In contrast, late Quaternary ice sheets grew as deep water, formed at high latitude, cooled. Because tectonic forcing and orbital forcing at low‐latitude primarily controlled production and temperature variations of this Warm Saline Deep Water, these influences largely dictated Tertiary ice volume fluctuations. Through the Tertiary, we infer ice volume fluctuations to be an important component of sea level history on timescales between 103 and 107 years.