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The eastern equatorial Pacific Ocean (EEP) upwelling system supports >10% of the present-day global ocean primary production, making it an important component in Earth's atmospheric and marine carbon budget. Traditionally, it has been argued that since intensification of Northern Hemisphere glaciation (iNHG, ~2.7 Ma), changes in EEP productivity have predominantly depended on trade wind strength-controlled upwelling intensity. An alternative hypothesis suggests that EEP productivity is primarily controlled by nutrient supply from the high southern latitudes via mode waters. Here we present new high-resolution data for the latest Pliocene/early Pleistocene from Ocean Drilling Program Site 849, located within the equatorial divergence system in the heart of the EEP upwelling regime. We use carbon isotopes in benthic and planktic foraminiferal calcite and sand accumulation rates to investigate glacial-interglacial (G-IG) productivity fluctuations between 2.65 and 2.4 Ma (marine isotope stages (MIS) G1 to 94). This interval includes MIS 100, 98, and 96, three large-amplitude glacials (~1‰ in benthic δ18O) representing the culmination of iNHG. Our results suggest that latest Pliocene/early Pleistocene G-IG productivity changes in the EEP were strongly controlled by nutrient supply from Southern Ocean-sourced mode waters. Our records show a clear G-IG cyclicity from MIS 100 onward with productivity levels increasing from full glacial conditions and peaking at glacial terminations. We conclude that enhanced nutrient delivery from high southern latitudes during full glacial conditions together with superimposed intensified regional upwelling toward glacial terminations strongly regulated primary productivity rates in the EEP from MIS 100 onward.

The amplitude of climatic change, as recorded in the benthic oxygen isotope record, has varied throughout geological time. During the late Pleistocene, changes in the atmospheric concentration of carbon dioxide (CO2) are an important control on this amplitude of variability. The contribution of CO2 to climate variability during the pre-Quaternary however is unknown. Here we present a new boron isotope-based CO2 record for the transition into the middle Miocene Climatic Optimum (MCO) between 15.5 and 17 Myr that shows pronounced variability between 300 ppm and 500 ppm on a roughly 100 kyr time scale during the MCO. The CO2 changes reconstructed for the Miocene are ~2 times larger in absolute terms (300 to 500 ppm compared to 180 to 280 ppm) than those associated with the late Pleistocene and ~15% larger in terms of climate forcing. In contrast, however, variability in the contemporaneous benthic oxygen isotope record (at ~1‰) is approximately two thirds the amplitude of that seen during the late Pleistocene. These observations indicate a lower overall sensitivity to CO2 forcing for Miocene (Antarctic only) ice sheets than their late Pleistocene (Antarctic plus lower latitude northern hemisphere) counterparts. When our Miocene CO2 record is compared to the estimated changes in contemporaneous δ18Osw (ice volume), they point to the existence of two reservoirs of ice on Antarctica. One of these reservoirs appears stable, while a second reservoir shows a level of dynamism that contradicts the results of coupled climate-ice sheet model experiments given the CO2 concentrations that we reconstruct.

Stable oxygen and carbon isotope (δ18O and δ13C) values measured in foraminiferal calcite are one of the primary tools used in paleoceanography. Diagenetic recrystallisation of foraminiferal calcite can act to reset primary isotopic values but its effects are typically poorly quantified. Here we test the impact of early stage diagenesis on stable isotope records generated from a suite of drill sites in the equatorial Pacific Ocean recovered during Ocean Drilling Program (ODP) Leg 199 and Integrated Ocean Drilling Program (IODP) Expedition 320. Our selected sites form paleowater- and burial-depth transects, with excellent stratigraphic control allowing us to confidently correlate our records. We observe large inter-site differences in the preservation state of benthic foraminiferal calcite, implying very different recrystallisation histories, but negligible inter-site offsets in benthic δ18O and δ13C values. We infer that diagenetic alteration of benthic foraminiferal calcite (in sedimentary oozes) must predominantly occur at shallow burial depths (

The strength and latitudinal position of the North Atlantic Current, NAC, determines the position of the Arctic front and heat transport to the high northern latitudes with potentially important consequences for Northern Hemisphere glaciation. A southward shift in the NAC and reduced poleward heat transport is hypothesized to have triggered the last major climate transition in Earth's history—late Pliocene intensification of Northern Hemisphere glaciation (iNHG). In turn, iNHG is hypothesized to have led to the amplification of climate variability on suborbital time scales. To date, however, only a handful of adequately resolved records are available to test these two hypotheses. Here we present a new late Pliocene to earliest Pleistocene record from Integrated Ocean Drilling Program Site U1313 (North Atlantic, 41°N; 2.9 to 2.4?Ma). We use Mg/Ca-derived paleotemperature records in planktic foraminiferal calcite to investigate changes in summer sea-surface temperatures (SST) on orbital and suborbital time scales. Our results call into question the suggestion that significant weakening and/or southward shift of the NAC served as a trigger for Northern Hemisphere cooling and intensified continental ice sheet growth across iNHG. In contrast to the late Pleistocene, during iNHG, we find that the position of the NAC and Arctic Front probably lay well to the north of Site U1313 and that the amplitude of suborbital SST variability did not change on glacial-interglacial time scales. Conservative estimates of Late Pliocene to earliest Pleistocene interglacial summer SSTs in our record are up to 3°C warmer than present, while glacial summer SSTs are only 2°C to 3°C cooler. In fact, our interglacial summer SSTs are remarkably similar to those of the mid-Pliocene. Our findings indicate that iNHG must have involved amplifying feedback mechanisms that are tightly coupled to ice sheet growth but that these processes were insufficiently developed by the late Pliocene/earliest Pleistocene to have triggered large amplitude changes in suborbital climate in the midlatitude North Atlantic.

A critical issue in many paleoceanographic studies is the potential of diagenetic processes to compromise the fidelity of geochemical proxies (e.g., δ18O) in deep-sea carbonate sediments and microfossils. A recently established strategy for dealing with this issue is to drill depth transects in anomalously shallowly buried and therefore presumably well-preserved target sediments. Here we report the results of an experiment designed to test the validity of this approach. We apply a simple one-dimensional numerical model to complex pore water chemical profiles (for Ca, Mg, Sr, 87Sr/86Sr, and δ18O) observed in a shallowly buried paleoceanographic transect of paleogene and Cretaceous age drilled on Blake Nose during Ocean Drilling Program Leg 171B. We show that it is possible to reproduce these complex pore water profiles simply by invoking (1) a deep-seated zone of chemical reaction well below the recovered sections and (2) minimal chemical alteration within the overlying recovered sediments but significant restriction of advective flow by embedded low-porosity layers. These results suggest that the sediments recovered during Ocean Drilling Program Leg 171B can be considered to be virtually chemically inert at their present burial depth (their carbonate recrystallization histories having been largely limited to events during early, ∼10 Ma timescale, burial. These findings support the strategy of selecting unusually shallowly buried sections for paleoceanographic transect drilling. Ironically, however, the very attraction of targeting such sections (the lack of overlying sediments) dictates that it is not possible to use pore water chemistry to constrain the early diagenetic histories of such sections because of the diffusion of the pore water signal through the sediment-water interface. Diffusional loss may also account for the absence of pore water chemical anomalies associated with the ooze-chalk transition at Blake Nose. However, our work suggests that traditional interpretations of this boundary as the inferred locus of maximum rates of carbonate recrystallization require reevaluation.

Mid-Cretaceous δ18O paleotemperatures are compared with temperatures predicted by an ocean general circulation model. The δ18O paleotemperatures, calculated assuming a δw of −1.0‰ (SMOW), are 5.6°C lower at low latitudes and 2.4°C higher at middle and high latitudes than the model-predicted, mixed-layer temperatures. The model-data comparison is improved after accounting for the spatial variability of δw and after considering the paleohabitats of the marine organisms from which δ18O values were measured. Paleotemperatures adjusted using the Broecker [1989] δw-S relationship and the model-predicted salinity are 3.4°C higher at low-latitude sites and 1.9°C lower at higher-latitude sites. Moreover, seven of nine low-latitude paleotemperatures intersect the model-predicted temperature within the upper 100 m of the water column, a depth consistent with the interpreted paleohabitat of the analyzed organisms. However, adjusted paleotemperatures for three of five Southern Hemisphere high-latitude locations are at least 9°C higher than the model-predicted temperatures for these locations. The model-data comparison suggests that mid-Cretaceous δ18O values are compatible with sea surface temperatures as high or higher than those simulated by an ocean general circulation model of the mid-Cretaceous with 4 × present-day atmospheric pCO2.

The onset of sustained Antarctic glaciation across the Eocene-Oligocene transition (EOT) marks a pivotal change in Earth's climate, but our understanding of this event, particularly the role of the carbon cycle, is limited. To help address this gap we present the following paleoceanographic proxy records from Ocean Drilling Program Site 1218 in the eastern equatorial Pacific (EEP): (1) stable isotope (δ18O and δ13C) records generated in epifaunal benthic foraminifera (Cibicidoides spp.) to improve (double the resolution) the previously published records; (2) δ18O and δ13C records measured on Oridorsalis umbonatus, a shallow infaunal species; and (3) a record of benthic foraminifera accumulation rate (BFAR). Our new isotope data sets confirm the existence at Site 1218 of a two-step δ18O increase. They also lend support to the hypothesized existence of a late Eocene transient δ18O increase and early Oligocene Oi-1a and Oi-1b glacial maxima. Our record of BFAR indicates a transient (∼500 kyr) twofold to threefold peak relative to baseline Oligocene values associated with the onset of Antarctic glaciation that we attribute to enhanced biological export production in the EEP. This takes the same general form as the history of opal accumulation in the Southern Ocean, suggesting strong high-to-low-latitude oceanic coupling. These findings appear to lend support to the idea that the EOT δ13C excursion is traceable to increased organic carbon (Corg) burial. Paradoxically, early Oligocene sediments in the EEP are extremely Corg-poor, and proxy records of atmospheric pCO2 indicate a transient increase associated with the EOT.

[1] Large-amplitude millennial-scale climate oscillations have been identified in late Pleistocene climate archives from around the world. These oscillations appear to be of larger amplitude during times of enlarged ice sheets. This observation suggests the existence of a relationship between large-amplitude millennial variations in climate and extreme glacial conditions and therefore that the emergence of millennial-scale climate variability may be linked to the Pliocene intensification of northern hemisphere glaciation (iNHG). Here we test this hypothesis using new late Pliocene high-resolution (∼400 year) records of ice-rafted debris deposition and stable isotopes in planktic foraminiferal calcite (Globigerinoides ruber) generated from Integrated Ocean Drilling Program Site U1313 in the subpolar North Atlantic (a reoccupation of the classic Deep Sea Drilling Project Site 607). Our records span marine oxygen isotope stages (MIS) 103–95 (∼2600 to 2400 ka), the first interval during iNHG (∼3.5 to 2.5 Ma) in which large-amplitude glacial-interglacial cycles and inferred sea level changes occur. Our records reveal small-amplitude variability at periodicities of ∼1.8 to 6.2 kyr that prevails regardless of (inter)glacial state with no significant amplification during the glacials MIS 100, 98, and 96. These findings imply that the threshold for the amplification of such variability to the proportions seen in the marine archive of the last glacial was not crossed during the late Pliocene and, in view of all available data, likely not until the Mid-Pleistocene Transition.

The tropics have played a central role in modulating Earth's climate throughout the Plio?Pleistocene, with tropical productivity fluctuations a key mechanism in the operation of the global carbon cycle and linkage of high? and low?latitude climates. Published records of tropical sea surface temperatures (SSTs) during the Plio?Pleistocene appear to vary primarily in tune with high?latitude climate on both orbital and secular timescales. However, contemporaneous changes in equatorial primary productivity are less well constrained, particularly at sites where climate is not dominated by upwelling or monsoon systems. Furthermore, the role of thermocline dynamics (tilt and mean depth changes) in forcing SST and productivity on orbital timescales remains uncertain. Here we report new, high?resolution calcareous nannofossil records from two Ocean Drilling Program sites in the western and eastern equatorial Pacific during marine isotope stages 95–101, about 2400–2600 kyr ago. Our records of paleoproductivity and nutricline depth reveal synchronous, large?amplitude glacial?interglacial productivity variations at both ends of the equatorial Pacific indicating (1) remote (high?latitude) forcing of primary productivity and (2) no primary role for east–west tilting of the equatorial Pacific thermocline, with important implications regarding the operation of El Nino–like dynamics in the Pliocene Pacific. Instead, the paleoproductivity variations and phase relationships that we document suggest the interaction of two mechanisms operating on obliquity timescales: a “bottom?up” forcing transmitted via the upwelling of high?latitude source waters in conjunction with the “top?down” forcing of atmospheric greenhouse gases.

[1] Suborbital variability in late Pleistocene records of ice-rafted debris and sea surface temperature in the North Atlantic Ocean appears most extreme during times of enlarged ice sheets with a well-constrained benthic oxygen isotope-defined “ice volume threshold” (δ18OT) for the “100 ka (inter)glacial” world. Information on climate instability for the earlier Pleistocene and late Pliocene is more fragmentary and/or of much lower temporal resolution, but the data available suggest similar behavior with δ18OT remaining more or less constant over the past 3000 ka. This finding is puzzling because it implies that ice rafting is highly sensitive to ice volume on short (suborbital/glacial-interglacial) time scales but not to the long-term changes in ice sheet composition associated with intensification of Northern Hemisphere glaciation (NHG). Here we report new high-resolution records of stable isotope change and ice rafting in the North Atlantic at Integrated Ocean Drilling Program Site U1308 (reoccupation of Deep Sea Drilling Project Site 609) during two glacials key to intensification of NHG (marine isotope stages G4, ∼2640 ka, and 100, ∼2520 ka). We find a pattern of suborbital ice rafting events showing clear evidence of threshold behavior. However, contrary to previous reports, we find that δ18OT for the late Pliocene is up to 0.45‰ Vienna Peedee belemnite (VPDB) lower than for the late Pleistocene. Using published Plio-Pleistocene global sea level records, we evaluate different potential explanations for this finding. We conclude that the observed Pliocene-Pleistocene offset in δ18OT is attributable to the existence of low-slung Pliocene ice sheets that flowed more readily than their late Pleistocene counterparts, associated with a smaller contemporaneous continental ice volume and less isotopically depleted ice.

Published stable isotope records in marine carbonate are characterized by a positive ?18O excursion associated with a negative ?13C shift during the early Maastrichtian. However, the cause and even the precise timing of these excursions remain uncertain. We have generated high-resolution foraminiferal stable isotope and grey scale records for the latest Campanian to early Maastrichtian (~73-68 Ma) at two Ocean Drilling Program sites, 525 (Walvis Ridge) and 690 (Weddell Sea). We demonstrate that the negative ?13C excursion is decoupled from the ?18O increase with a lag of about 600 kyr. Our ?13C records (both planktic and benthic) show an amplitude for the negative excursion of 0.7 ‰ that falls between about 72.1 and 70.7 Ma. Our planktic ?18O records indicate an overall increase of 1.2 ‰ from 73 to 68 Ma at Site 690, whereas at Site 525 they record a slightly smaller increase (~1 ‰) that peaks around 70.1 Ma with decreasing values thereafter. Our benthic ?18O data indicate an increase of ~1.5 ‰ at Site 525 and ~0.7 ‰ at Site 690 between about 71.4 and 69.9 Ma. Benthic ?18O values show different baseline values at the two sites before and after the excursion but the larger increase at Site 525 means that the values attained at the peak of the excursion are similar at the two sites. We interpret this observation in terms of water mass changes. The excursion is interpreted to reflect a cooling of bottom waters in response to the strengthening contribution of intermediate- to deep-water production in the high southern latitudes rather than increased ice volume. The associated carbon-cycle perturbations that we observe are interpreted to reflect a weakening of surface-water stratification and increased productivity, as supported by our grey value data.

A late Albian–early Cenomanian record (~103.3 to 99.0 Ma), including organic-rich deposits and a ?13C increase associated with oceanic anoxic event 1d (OAE 1d), is described from Ocean Drilling Program sites 1050 and 1052 in the subtropical Atlantic. Foraminifera are well preserved at these sites. Paleotemperatures estimated from benthic ?18O values average ~14°C for middle bathyal Site 1050 and ~17°C for upper bathyal Site 1052, whereas surface temperatures are estimated to have ranged from 26°C to 31°C at both sites. Among planktonic foraminifera, there is a steady balance of speciation and extinction with no discrete time of major faunal turnover. OAE 1d is recognized on the basis of a 1.2‰ ?13C increase (~100.0–99.6 Ma), which is similar in age and magnitude to ?13C excursions documented in the North Atlantic and western Tethys. Organic-rich “black shales” are present throughout the studied interval at both sites. However, deposition of individual black shale beds was not synchronous between sites, and most of the black shale was deposited before the OAE 1d ?13C increase. A similar pattern is observed at the other sites where OAE 1d has been recognized indicating that the site(s) of excess organic carbon burial that could have caused the ?13C increase has (have) yet to be found. Our findings add weight to the view that OAEs should be chemostratigraphically (?13C) rather than lithostratigraphically defined.

Oceanic anoxic event 2 (OAE-2) occurring during the Cenomanian/Turonian (C/T) transition is evident from a globally recognized positive stable carbon isotopic excursion and is thought to represent one of the most extreme carbon cycle perturbations of the last 100 Myr. However, the impact of this major perturbation on and interaction with global climate remains unclear. Here we report new high-resolution records of sea surface temperature (SST) based on TEX86 and ?18O of excellently preserved planktic foraminifera and stable organic carbon isotopes across the C/T transition from black shales located offshore Suriname/French Guiana (Demerara Rise, Ocean Drilling Program Leg 207 Site 1260) and offshore Senegal (Cape Verde Basin, Deep Sea Drilling Project Leg 41 Site 367). At Site 1260, where both SST proxy records can be determined, a good match between conservative SST estimates from TEX86 and ?18O is observed. We find that late Cenomanian SSTs in the equatorial Atlantic Ocean (?33°C) were substantially warmer than today (~27°–29°C) and that the onset of OAE-2 coincided with a rapid shift to an even warmer (~35°–36°C) regime. Within the early stages of the OAE a marked (~4°C) cooling to temperatures lower than pre-OAE conditions is observed. However, well before the termination of OAE-2 the warm regime was reestablished and persisted into the Turonian. Our findings corroborate the view that the C/T transition represents the onset of the interval of peak Cretaceous warmth. More importantly, they are consistent with the hypotheses that mid-Cretaceous warmth can be attributed to high levels of atmospheric carbon dioxide (CO2) and that major OAEs were capable of triggering global cooling through the negative feedback effect of organic carbon-burial-led CO2 sequestration. Evidently, however, the factors that gave rise to the observed shift to a warmer climate regime at the onset of OAE-2 were sufficiently powerful that they were only briefly counterbalanced by the high rates of carbon burial attained during even the most extreme interval of organic carbon burial in the last 100 Myr.

[1] Until recently, very few high-quality deep ocean sedimentary sections of Eocene age have been available. Consequently, our understanding of Eocene paleoceanography has become heavily reliant on “composite” records patched together from multiple sites in different ocean basins and generated using multiple taxa (potential sources of “local” noise in the global signal). Here we test the reliability of the early to middle Eocene composite δ18O and δ13C stratigraphies (Zachos et al., 2001) by generating new monospecific records in benthic foraminiferal calcite from a single locality, Demerara Rise, in the tropical western Atlantic (Ocean Drilling Program Leg 207). We present new stable isotope correction factors for commonly used Eocene benthic foraminiferal species. We find that interspecies isotopic offsets are constant across the isotopic range, supporting the notion that the inconstant intertaxa offsets reported elsewhere result from mixing species within genera. In general, the δ18O stratigraphy from Demerara Rise supports the validity of the Eocene δ18O composite, while revealing a temporary warming punctuating middle Eocene cooling. This warming may correspond to the so-called “Middle Eocene Climatic Optimum” previously documented in the Southern Ocean. The composite and Demerara Rise records for δ13C differ substantially. By removing the intersite and intertaxa sources of uncertainty in δ13C, we obtain a clearer picture of carbon cycling during the Eocene. Secular change in interocean δ13C gradients through the Eocene reveals that intervals of climatic warmth (especially the early Eocene) are associated with very small water mass ageing gradients.

Well-preserved nannofossil and stable isotope records from the mid-Cretaceous of Ocean Drilling Project Leg 171B (western North Atlantic) indicate cyclical, productivity-based variations in surface water characteristics, suggesting orbitally paced changes in upwelling intensity and the strength of deep mixing associated with oceanic anoxic event (OAE)1d. Paleontologic and isotopic evidence suggest that collapse of upper water column stratification associated with OAE1d was preceded by approximately 1 m.y. of progressively decreasing water column stability and increasing surface water fertility. Thirteen species went extinct during a short (ca. 200 k.y.) interval associated with OAE1d. Nine of these have morphological characters suggesting adaptation to specific depths in the photic zone. Simultaneous extinction of these depth-zoned species suggests that stratification collapse disrupted the habitat space for these specialized forms, resulting in extinction. A cyclostratigraphic model provides age estimates of several important nannofossil datums including the first appearances of Eiffellithus turriseiffelii (100.95 Ma) and Corollithion kennedyi (99.55 Ma).

Paired benthic foraminiferal trace metal and stable isotope records have been constructed from equatorial Pacific Ocean Drilling Program Site 1218. The records include the two largest abrupt (