Benthic foraminiferal assemblage changes during interstadial 4 to 8 in ODP Site 146-893A, Santa Barbara Basin

Benthic foraminiferal assemblages from Santa Barbara Basin exhibit major faunal and ecological switches associated with late Quaternary millennial- to decadal-scale global climate oscillations. Repeated turnovers of entire faunas occurred rapidly (<40-400 yr) without extinction or speciation in conjunction with Dansgaard-Oeschger shifts in thermohaline circulation, ventilation, and climate, confirming evolutionary model predictions of Roy et al. Consistent faunal successions of dysoxic taxa during successive interstadials reflect the extreme sensitivity and adaptation of the benthic ecosystem to the rapid environmental changes that marked the late Quaternary and possibly other transitional intervals in the history of the Earth's ocean-atmosphere-cryosphere system. These data support the hypothesis that broad segments of the biosphere are well adapted to rapid climate change.

1.2 Ma stacked benthic foraminiferal carbon isotope record

Pleistocene stable carbon isotope (d13C) records from surface and deep dwelling foraminifera in all major ocean basins show two distinct long-term carbon isotope fluctuations since 1.00 Ma. The first started around 1.00 Ma and was characterised by a 0.35 per mil decrease in d13C values until 0.90 Ma, followed by an increase of 0.60 per mil lasting until 0.50 Ma. The subsequent fluctuation started with a 0.40 per mil decrease between 0.50 and 0.25 Ma, followed by an increase of 0.30 per mil between 0.25 and 0.10 Ma. Here, we evaluate existing evidence and various hypotheses for these global Pleistocene d13C fluctuations and present an interpretation, where the fluctuations most likely resulted from concomitant changes in the burial fluxes of organic and inorganic carbon due to ventilation changes and/or changes in the production and export ratio. Our model indicates that to satisfy the long-term 'stability' of the Pleistocene lysocline, the ratio between the amounts of change in the organic and inorganic carbon burial fluxes would have to be close to a 1:1 ratio, as deviations from this ratio would lead to sizable variations in the depth of the lysocline. It is then apparent that the mid-Pleistocene climate transition, which, apart from the glacial cycles, represents the most fundamental change in the Pleistocene climate, was likely not associated with a fundamental change in atmospheric pCO2. While recognising that high frequency glacial/interglacial cycles are associated with relatively large (100 ppmv) changes in pCO2, our model scenario (with burial changes close to a 1:1 ratio) produces a maximum long-term variability of only 20 ppmv over the fluctuation between 1.00 and 0.50 Ma.

Mg/Ca and stable isotope record of benthic foraminifera of the Paleocene-Eocene Thermal Maximum

A rapid increase in greenhouse gas levels is thought to have fueled global warming at the Paleocene-Eocene Thermal Maximum (PETM). Foraminiferal magnesium/calcium ratios indicate that bottom waters warmed by 4° to 5°C, similar to tropical and subtropical surface ocean waters, implying no amplification of warming in high-latitude regions of deep-water formation under ice-free conditions. Intermediate waters warmed before the carbon isotope excursion, in association with downwelling in the North Pacific and reduced Southern Ocean convection, supporting changing circulation as the trigger for methane hydrate release. A switch to deep convection in the North Pacific at the PETM onset could have amplified and sustained warming.

Sr/Ca ratios of Cenozoic benthic foraminifera

A Cenozoic multi-species record of benthic foraminiferal calcite Sr/Ca has been produced and is corrected for interspecific offsets (typically less than 0.3 mmol/mol) and for the linear relationship between decreasing benthic foraminiferal Sr/Ca and increasing water depth. The water depth correction, determined from Holocene, Late Glacial Maximum and Eocene paleowater-depth transects, is ~0.1 mmol/mol/km. The corrected Cenozoic benthic foraminiferal Sr/Ca record ranges from 1.2 to 2.0 mmol/mol, and has been interpreted in terms of long-term changes in seawater Sr/Ca, enabling issues related to higher-resolution variability in Sr/Ca to be ignored. We estimate that seawater Sr/Ca was ~1.5 times modern values in the late Cretaceous, but declined rapidly into the Paleogene. Following a minimum in the Eocene, seawater Sr/Ca increased gradually through to the present day with a minimum superimposed on this trend centered in the late Miocene. By assuming scenarios for changing seawater calcium concentration, and using published carbonate accumulation rate data combined with suitable values for Sr partition coefficients into carbonates, the seawater Sr/Ca record is used to estimate global average river Sr fluxes. These fluxes are used in conjunction with the seawater strontium isotope curve and estimates of hydrothermal activity/tectonic outgassing to calculate changes in global average river 87Sr/86Sr through the Cenozoic. The absolute magnitude of Sr fluxes and isotopic compositions calculated in this way are subject to relatively large uncertainties. Nevertheless, our results suggest that river Sr flux increased from 35 Ma to the present day (roughly two-fold) accompanied by an overall increase in 87Sr/86Sr (by ~0 to 0.001). Between 75 and 35 Ma, river 87Sr/86Sr also increased (by ~0.001 to 0.002) but was accompanied by a decrease (two- to three-fold) in river Sr flux.

Early Eocene benthic foraminifera of DSDP Hole 48-401

Within the last decade, several early Eocene hyperthermals have been detected globally. These transient warming events have mainly been characterized geochemically - using stable isotopes, carbonate content measurements or XRF core scanning - yet detailed micropaleontological records are sparse, limiting our understanding of the driving forces behind hyperthermals and of the contemporaneous paleoceanography. Here, detailed geochemical and quantitative benthic foraminiferal records are presented from lower Eocene pelagic sediments of Deep Sea Drilling Project Site 401 (Bay of Biscay, northeast Atlantic). In calcareous nannofossil zone NP11, several clay-enriched levels correspond to negative d13C and d18O bulk-rock excursions with amplitudes of up to ~0.75 per mil, suggesting that significant injections of 12C-enriched greenhouse gasses and small temperature rises took place. Coeval with several of these hyperthermal events, the benthic foraminiferal record reveals increased relative abundances of oligotrophic taxa (e.g. Nuttallides umbonifera) and a reduction in the abundance of buliminid species followed by an increase of opportunistic taxa (e.g. Globocassidulina subglobosa and Gyroidinoides spp.). These short-lived faunal perturbations are thought to be caused by reduced seasonality of productivity resulting in a decreased Corg flux to the seafloor. Moreover, the sedimentological record suggests that an enhanced influx of terrigenous material occurred during these events. Additionally, the most intense d13C decline (here called level d) gives rise to a small, yet pronounced long-term shift in the benthic foraminiferal composition at this site, possibly due to the reappraisal of upwelling and the intensification of bottom water currents. These observations imply that environmental changes during (smaller) hyperthermal events are also reflected in the composition of deep-sea benthic communities on both short (<100 kyr) and longer time scales. We conclude that the faunal patterns of the hyperthermals observed at Site 401 strongly resemble those observed in other deep-sea early Paleogene hyperthermal deposits, suggesting that similar processes have driven them.

Benthic foraminifera extinctions in the Cenozoic record

In the late Pliocene-middle Pleistocene a group of 95 species of elongate, cylindrical, deep-sea (lower bathyal-abyssal) benthic foraminifera became extinct. This Extinction Group (Ext. Gp), belonging to three families (all the Stilostomellidae and Pleurostomellidae, some of the Nodosariidae), was a major component (20-70%) of deep-sea foraminiferal assemblages in the middle Cenozoic and subsequently declined in abundance and species richness before finally disappearing almost completely during the mid-Pleistocene Climatic Transition (MPT). So what caused these declines and extinction? In this study 127 Ext. Gp species are identified from eight Cenozoic bathyal and abyssal sequences in the North Atlantic and equatorial Pacific Oceans. Most species are long-ranging with 80% originating in the Eocene or earlier. The greatest abundance and diversity of the Ext. Gp was in the warm oceanic conditions of the middle Eocene-early Oligocene. The group was subjected to significant changes in the composition of the faunal dominants and slightly enhanced species turnover during and soon after the rapid Eocene-Oligocene cooling event. Declines in the relative abundance and flux of the Ext. Gp, together with enhanced species loss, occurred during middle-late Miocene cooling, particularly at abyssal sites. The overall number of Ext. Gp species present began declining earlier at mid abyssal depths (in middle Miocene) than at upper abyssal (in late Pliocene-early Pleistocene) and then lower bathyal depths (in MPT). By far the most significant Ext. Gp declines in abundance and species loss occurred during the more severe glacial stages of the late Pliocene-middle Pleistocene. Clearly, the decline and extinction of this group of deep-sea foraminifera was related to the function of their specialized apertures and the stepwise cooling of global climate and deep water. We infer that the apertural modifications may be related to the method of food collection or processing, and that the extinctions may have resulted from the decline or loss of their specific phytoplankton or prokaryote food source, that was more directly impacted than the foraminifera by the cooling temperatures.

Neodymium isotopes from fossil fish teeth and detrital sediment, and oxygen isotopes from benthic forams from ODP Hole 119-738 B

Ephemeral polar glaciations during the middle-to-late Eocene (48-34 Ma) have been proposed based on far-field ice volume proxy records and near-field glacigenic sediments, although the scale, timing, and duration of these events are poorly constrained. Here we confirm the existence of a transient cool event within a new high-resolution benthic foraminiferal d18O record at Ocean Drilling Program (ODP) Site 738 (Kerguelen Plateau; Southern Ocean). This event, named the Priabonian oxygen isotope maximum (PrOM) Event, lasted ~140 kyr and is tentatively placed within magnetochron C17n.1n (~37.3 Ma) based on the correlation to ODP Site 689 (Maud Rise, Southern Ocean). A contemporaneous change in the provenance of sediments delivered to the Kerguelen Plateau occurs at the study site, determined from the <63 µm fraction of decarbonated and reductively leached sediment samples. Changes in the mixture of bottom waters, based on fossil fish tooth epsilon-Nd, were less pronounced and slower relative to the benthic d18O and terrigenous epsilon-Nd changes. Terrigenous sediment epsilon-Nd values rapidly shifted to less radiogenic signatures at the onset of the PrOM Event, indicating an abrupt change in provenance favoring ancient sources such as the Paleoproterozoic East Antarctic craton. Bottom water epsilon-Nd reached a minimum value during the PrOM Event, although the shift begins much earlier than the terrigenous epsilon-Nd excursion. The origin of the abrupt change in terrigenous sediment provenance is compatible with a change in Antarctic terrigenous sediment flux and/or source as opposed to a reorganization of ocean currents. A change in terrigenous flux and/or source of Antarctic sediments during the oxygen isotope maximum suggests a combination of cooling and ice growth in East Antarctica during the early late Eocene.

Total number of benthic foraminifera in sediments below the K/T boundary in the Atlantic and Pacific Ocean

Most species of Late Cretaceous deep-sea benthic foraminifera are believed to be cosmopolitan and therefore to exhibit only minor biogeographical differences. In this preliminary report, six Deep Sea Drilling Project (DSDP) sites from different oceans, paleolatitudes, and paleodepths were analyzed for terminal Cretaceous abyssal-bathyal benthic foraminifera in order to investigate their assumed cosmopolitan distribution and the question of whether different faunal compositions are related to time, different paleolatitudes, and/or different paleodepths. The material studied was obtained from the low-latitude Site 465 (Pacific Ocean), and the intermediate-latitude Sites 384 (North Atlantic) and 356, 516, 525, and 527 (South Atlantic). The material analyzed represents a time slice encompassing the last 20-50 k.y. of the Cretaceous. The faunas contain numerous "Velasco-type" species, such as Gavelinella beccariiformis (White), Cibicidoides velascoensis (Cushman), Nuttallides truempyi (Nuttall), Gaudryina pyramidata Cushman, and various gyroidinoids and buliminids. The results contradict the general assumption of the cosmopolitan nature of Late Cretaceous deep-sea benthic foraminifera advocated in the literature. Only about 9% of the taxa identified were found to be truly "cosmopolitan" through their occurrence at all the sites analyzed. On the basis of correspondence analysis and relative abundance data, three assemblages and three subassemblages were recognized: (1) a bathyal-abyssal assemblage [Nuttallinella sp. A, Cibicidoides hyphalus (Fisher), Valvalabamina sp. evolute form, and Gyroidinoides spp.] at the South Atlantic Sites 356, 516, 525, and 527, divided into three subassemblages, namely (a) a middle bathyal subassemblage [Eouvigerina subsculptura McNeil and Caldwell, Truaxia aspera (Cushman), and G. pyramidata] at Sites 516 and 525, (b) a lower bathyal subassemblage [Osangularia? sp., Pyramidina rudita (Cushman and Parker), and Quadrimorphina camerata (Brotzen)] at Site 356, and (c) an abyssal subassemblage [Gyroidinoides sp. C, Hyperammina-Bathysiphon, Gyroidinoides beisseli (White), and Globorotalites sp. B] at Site 527; (2) an abyssal assemblage [Buliminella cf. plana (Cushman and Parker) and Bulimina incisa Cushman] at the North Atlantic Site 384; and (3) a middle bathyal assemblage [Vulvulina sp. A, Osangularia navarroana (Cushman), Alabamina? sp., Bulimina velascoensis (Cushman), Spiroplectammina spp. calcareous forms, and Bulimina trinitatensis Cushman and Jarvis] at the Pacific Site 465.

Sedimentology and Cd/Ca ratios of benthic foraminifera from ODP Site 172-1060 in the western Nort Atlantic

The Western Boundary Undercurrent (WBUC), off eastern America, is an important component of the Atlantic Meridional Overturning circulation and is the principal route for southward transport of North Atlantic waters and southward return of Southern Source Water (SSW). Here a direct flow speed proxy (mean grain size of the sortable silt) is used to infer the vigour of flow of the palaeo-WBUC at Blake Outer Ridge, (ODP Site 1060, depth 3481 m) during Marine Isotope Stage (MIS) 3. The overall shape of the flow speed proxy record shows a complex pattern of variability, with generally more vigorous flow and larger-scale flow variations between 35 and 60 ka than in the younger part of MIS 3 and MIS 2 (b35 ka). Six events of reduced bottom flow vigour (Slow Events, SEs) occur. These appear uncorrelated with Heinrich events, but are instead synchronous with the warming phases of Antarctic Warm Events A-1 to A-4 (with one new one, A-1a and one poorly defined, 'A-0'). This indicates that Antarctic climate exerts a stronger control on deep flow vigour in the North Atlantic during MIS 3 than Northern Hemisphere climate. The correspondence of SEs with Antarctic warming suggests a weaker WBUC flow due to reduced volume flux at SSW source or reduced SSW density. Because the variability of the lower limb of the WBUC was not connected to sharp North Atlantic changes in temperature, it is unlikely that the Dansgaard/Oeschger cycles were associated with a mode of MOC variation involving wholeocean overturn, but more likely with perturbations of only the shallow Glacial Gulf Stream-Glacial Northern Source Intermediate Water cell. Nutrient proxies (benthic carbon isotopes and Cd/Ca of Uvigerina peregrina) at this site show similar trends to the GRIP delta18O record. This correlation has previously been attributed mainly to hydrographic and flow changes but is here shown to be better explained by variations in surface ocean productivity and subsequent decomposition of 12C rich organic material on the sea floor.