Atlantic Ocean Drilling Program Site Research Articles

Carbon Isotopic Fractionation of Alkenones and Gephyrocapsa Coccoliths Over the Late Quaternary (Marine Isotope Stages 12–9) Glacial‐Interglacial Cycles at the Western Tropical Atlantic

AbstractThe sensitivity of coccolithophores to changing CO2 and its role modulating cellular photosynthetic carbon isotopic fractionation (εp) is crucial to understand the future adaptation of these organisms to higher CO2 world and to assess the reliability of εp for past CO2 estimation. Here, we present εp measured on natural fossil samples across the glacial‐interglacial (G‐I) CO2 variations of marine isotope stages 12 to 9 interval (454–334 ka) at the western tropical Atlantic Ocean Drilling Program Site 925 together with a set of organic and inorganic geochemical, micropaleontological and morphometrical data from Gephyrocapsa coccoliths in the same samples. The ∼2‰ variation in εp is significantly correlated with the CO2[aq] concentrations calculated from assumption of air‐sea equilibrium with measured ice core pCO2 concentrations. The sensitivity of εp to CO2[aq] is similar to that derived from a multiple regression model of culture observations and is not well simulated with the classical purely diffusive model of algal CO2 acquisition. The measured range of Gephyrocapsa cell sizes is insufficient to explain the non‐CO2 effects on εp at this location, via either direct size effect or growth rate correlated to cell size. Primary productivity, potentially triggered by shifting growth rates and light levels, may also affect εp. Proposed productivity proxies % Florisphaera profunda and the ratio between the C37 to C38.et alkenone (C37/C38.et ratio) both correlates modestly with the non‐CO2 effects on εp. When the observed G‐I εp to CO2 sensitivity at this site is used to estimate pCO2 from εp since the Miocene, the inferred pCO2 declines are larger in amplitude compared to that calculated from a theoretical εp diffusive model. We find that oxygen and carbon stable isotope vital effects in the near monogeneric‐separated Gephyrocapsa coccoliths (respectively Δδ18OGephyrocapsa–Trilobatus sacculifer and εcoccolith) are coupled through the time series, but the origins of these vital effects are not readily explained by existing models.

Changes in tropical Atlantic surface-water environments inferred from late Albian planktic foraminiferal assemblages (ODP Site 1258, Demerara Rise)

Abundance patterns of planktic and benthic foraminifera from the middle to upper Albian of tropical Atlantic Ocean Drilling Program Site 1258 (Demerara Rise) display a pronounced shift between two clearly distinguishable assemblages: (1) a foraminiferal fauna dominated by coiled taxa, which is inferred to represent a deep and/or weak oxygen minimum zone, and (2) abundant clavate and biserial taxa, inferred to record a shallow and/or strong oxygen minimum zone at Demerara Rise. The entire sequence is rich in organic matter (3–6 wt% TOC) of a marine phytoplankton origin (δ13C ∼ −28‰), consistent with a highly productive region. Both the high productivity and changes in foraminiferal assemblages are consistent with changes in the mean latitudinal position of the Intertropical Convergence Zone (ITCZ) over the South American continent. Movements of the ITCZ are proposed to reduce the amount of runoff during the 'clavate-dominated' cycles, leading to the intensification of upwelling and therefore a stronger oxygen minimum zone compared to the intervals dominated by coiled foraminifera. We have previously found a similar hydrographic cycle in Turonian to Coniacian sections from the same area suggesting that this climate cyclicity and palaeogeography was long-lived off NE South America (∼15 million years). Due to the stratigraphic position of the interval with high abundances of clavate planktic foraminifera and the inferred strong oxygen minimum zone, it is tentatively proposed that this interval represents a local expression of the OAE 1c event within the long-living Demerara Rise upwelling system.

Southeastern Atlantic deep-water evolution during the late-middle Eocene to earliest Oligocene (Ocean Drilling Program Site 1263 and Deep Sea Drilling Project Site 366)

Comparison of new benthic foraminiferal δ18O and δ13C records from Ocean Drilling Program (ODP) Site 1263 (Walvis Ridge, southeast Atlantic, 2100 m paleodepth) and Deep Sea Drilling Project (DSDP) Site 366 (Sierra Leone Rise, eastern equatorial Atlantic, 2200–2800 m paleodepth) with published data from Atlantic and Southern Ocean sites provides the means to reconstruct the development of deep-water circulation in the southeastern Atlantic from the late-middle Eocene to the earliest Oligocene. Our comparison shows that in the late-middle Eocene (ca. 40 Ma), the South Atlantic was characterized by a homogeneous thermal structure. Thermal differentiation began ca. 38 Ma. By 37.6 Ma, Site 1263 was dominated by Southern Component Water; at the same time, warm saline deep water filled the deeper South Atlantic (recorded at southwest Atlantic ODP Site 699, paleodepth 3400 m, and southeast Atlantic ODP Site 1090, paleodepth 3200 m). The deep-water source to eastern equatorial Site 366 transitioned to Northern Component Water ca. 35.6–35 Ma. Progressive cooling at Site 1263 during the middle to late Eocene and deep-water thermal stratification in the South Atlantic may be attributed at least in part to the gradual deepening and strengthening of the proto–Antarctic Circumpolar Current from the late-middle Eocene to the earliest Oligocene, as the Drake and Tasman gateways opened. Our isotopic comparisons across depth and latitude provide evidence of the development of deep-water circulation similar to modern-day Atlantic Meridional Overturning Circulation.

Evaluating climatic response to external radiative forcing during the late Miocene to early Pliocene: New perspectives from eastern equatorial Pacific (IODP U1338) and North Atlantic (ODP 982) locations

Orbital-scale climate variability during the latest Miocene-early Pliocene is poorly understood due to a lack of high-resolution records spanning 8.0–3.5 Ma, which resolve all orbital cycles. Assessing this variability improves understanding of how Earth's system sensitivity to insolation evolves and provides insight into the factors driving the Messinian Salinity Crisis (MSC) and the Late Miocene Carbon Isotope Shift (LMCIS). New high-resolution benthic foraminiferal Cibicidoides mundulus δ18O and δ13C records from equatorial Pacific International Ocean Drilling Program Site U1338 are correlated to North Atlantic Ocean Drilling Program Site 982 to obtain a global perspective. Four long-term benthic δ18O variations are identified: the Tortonian-Messinian, Miocene-Pliocene, and Early-Pliocene Oxygen Isotope Lows (8–7, 5.9–4.9, and 4.8–3.5 Ma) and the Messinian Oxygen Isotope High (MOH; 7–5.9 Ma). Obliquity-paced variability dominates throughout, except during the MOH. Eleven new orbital-scale isotopic stages are identified between 7.4 and 7.1 Ma. Cryosphere and carbon cycle sensitivities, estimated from δ18O and δ13C variability, suggest a weak cryosphere-carbon cycle coupling. The MSC termination coincided with moderate cryosphere sensitivity and reduced global ice sheets. The LMCIS coincided with reduced carbon cycle sensitivity, suggesting a driving force independent of insolation changes. The response of the cryosphere and carbon cycle to obliquity forcing is established, defined as Earth System Response (ESR). Observations reveal that two late Miocene-early Pliocene climate states existed. The first is a prevailing dynamic state with moderate ESR and obliquity-driven Antarctic ice variations, associated with reduced global ice volumes. The second is a stable state, which occurred during the MOH, with reduced ESR and lower obliquity-driven variability, associated with expanded global ice volumes.

Re-evaluation of the age model for North Atlantic Ocean Site 982 – arguments for a return to the original chronology

Abstract. Recently, the veracity of the published chronology for the Pliocene section of North Atlantic Ocean Drilling Program Site 982 was called into question. Here, we examine the robustness of the original age model as well as the proposed age model revision. The proposed revision is predicated on an apparent misidentification of the depth to the Gauss–Matuyama (G/M) polarity chronozone reversal boundary (2.581 Ma) based on preliminary shipboard paleomagnetic data, and offers a new chronology that includes a hiatus between ~3.2 and 3 Ma. However, an even more accurate shore-based, u-channel-derived polarity chronozone stratigraphy for the past ~2.7 Ma supports the shipboard composite stratigraphy and demonstrates that the original estimate of the depth of the G/M reversal in the Site 982 record is correct. Thus, the main justification forwarded to support the revised chronology no longer exists. We demonstrate that the proposed revision results in a pronounced anomaly in sedimentation rates proximal to the proposed hiatus, erroneous assignment of marine-isotope stages in the Site 982 Pliocene benthic stable oxygen isotope stratigraphy, and a markedly worse correlation of proxy records between this site and other regional paleoclimate data. We conclude that the original chronology for Site 982 is a far more accurate age model than that which arises from the published revision. We strongly recommend the use of the original chronology for all future work at Site 982.

Calcareous nannofossil changes during the Mid-Pleistocene Revolution: Paleoecologic and paleoceanographic evidence from North Atlantic Site 980/981

New quantitative data on calcareous nannofossil assemblages through 583–1138 thousand years ago (kyr) have been acquired in high sample resolution (2–3ky) at northern Atlantic ODP (Ocean Drilling Program) Site 980/981. Assemblage composition reflects a temperate paleobioprovince suggesting significant influx of warm North Atlantic Current at the site location during the mid-Pleistocene transition, between Marine Isotope Stage (MIS) 35 and 15.Three intervals have been described based on changes in nannofossil composition and abundance fluctuations of taxa. Reticulofenestrids are abundant just in the first interval (MIS 35–22), which is also characterised by the absence of Gephyrocapsa (>4μm) and C. pelagicus s.l., the latter occurring in upper MIS 24 only. The second interval corresponds to the expanded warm MIS 21 that is accompanied by high carbonate content, absence of both nannofossil reworking and ice rafted debris (IRD), higher abundance of C. leptoporus (5–8μm) and H. carteri, and absence of C. pelagicus s.l. and medium gephyrocapsids. The third interval (MIS 20–15) records the occurrence of C. pelagicus s.l. and medium Gephyrocapsa, generally higher abundance and higher amplitude fluctuations of helicoliths, and absence of reticulofenestrids. The major changes in nannofossil assemblages seem to be the result of combined paleoenvironmental factors related to sea surface water features (temperature and nutrient availability, sea-ice cover) and possibly to deep and intermediate water production of both Northern and Southern Hemisphere origin during glacial and interglacial phases. Complex inter-relationships between environmental parameters and evolutionary/ecological factors are inferred to explain the patterns of Coccolithus pelagicus subspecies, whose distribution and abundance fluctuations are compared with the available patterns of Turborotalia quinqueloba and Neogloboquadrina pachyderma (left coiled) at Site 980 and with the C. pelagicus subspecies patterns from North Atlantic DSDP (Deep Sea Drilling Project) Site 607 and South Atlantic ODP Site 1090.Data collected provide additional information on stratigraphic distribution of selected taxa; the First Occurrence (FO) of Gephyrocapsa omega is confirmed as a diachronous event, even in the Atlantic Ocean, while the First and Last Common Occurrence (FCO, LCO) of R. asanoi appear as diachronous and synchronous events, respectively, on a global scale.

Atlantic overturning circulation and Agulhas leakage influences on southeast Atlantic upper ocean hydrography during marine isotope stage 11

Climate dynamics during the marine isotope stage (MIS) 11 interglacial may provide information about how the climate system will evolve under the conditions of low-amplitude orbital forcing that are also found during the late Holocene. New stable isotope and alkenone data are presented from southeast Atlantic Ocean Drilling Program Site 1085, providing detailed information on interglacial climate evolution and the impacts of Atlantic meridional overturning circulation (MOC) and Agulhas leakage on the regional upper ocean hydrography. The data suggest that although warm surface ocean conditions were maintained at approximate Holocene levels for 40,000 years during MIS 11, subsurface temperature and salinity recorded by deeper-dwelling planktonic foraminifera species were maintained at their highest values for only 7000–8000 years. Surface water temperature and salinity data suggest that the interocean exchange of warm, salty waters into the southeast Atlantic Ocean was directly related to changes in the activity of the MOC during the study interval. Specifically, transient regional warming events during periods of weakened overturning circulation may have been amplified by the continuous interocean exchange of warm, salty Indian Ocean waters that primed the MOC for abrupt resumptions into a vigorous mode of operation. Conversely, a peak in interocean exchange at the end of the MIS 11 interglacial optimum may reflect enhanced trade wind forcing of surface waters whose export to the North Atlantic Ocean could have contributed to renewed ice sheet buildup during the MIS 11 to 10 glacial inception.

Influences of High‐ and Low‐Latitude Processes on African Terrestrial Climate: Pleistocene Eolian Records from Equatorial Atlantic Ocean Drilling Program Site 663

High‐ and low‐latitude forcing of terrestrial African paleoclimate variability is demonstrated using 900 ka eolian and biogenic component records from Ocean Drilling Program site 663 in the eastern equatorial Atlantic. Terrigenous (eolian dust) and phytolith (savannah grass cuticle) accumulation rate records vary predominantly at 100 and 41 kyr periodicities and spectral phase estimates implicate high‐latitude forcing. The abundance of freshwater diatoms (Melosira) transported from dry African lake beds varies coherently at 23–19 kyr orbital periodicities and at a phasing which implicates low‐latitude precessional monsoon forcing. Modeling studies demonstrate that African climate is sensitive to both high‐ and low‐latitude boundary conditions. African monsoon intensity is modulated by direct insolation variations due to orbital precession, whereas remote high‐latitude forcing can be related to cool North Atlantic sea surface temperatures (SSTs) which promote African aridity and enhance dust‐transporting wind speeds. The site 663 terrigenous and phytolith records covary with North Atlantic SST variability at 41 °N (site 607). We suggest that Pleistocene African climate has responded to both high‐latitude North Atlantic SST variability as well as low‐latitude precessional monsoon forcing; the high‐latitude influence dominates the sedimentary record. Prior to circa 2.4 Ma, terrigenous variations occurred primarily at precessional periodicities (23–19 kyr), indicating that African climate was largely controlled by low‐latitude insolation variations prior to the onset of high‐amplitude glacial‐interglacial climate change.

Interrelationships between the seasonal sea ice and biological regimes

New quantitative data on calcareous nannofossil assemblages through 583–1138 thousand years ago (kyr) have been acquired in high sample resolution (2–3 ky) at northern Atlantic ODP (Ocean Drilling Program) Site 980/981. Assemblage composition reflects a temperate paleobioprovince suggesting significant influx of warm North Atlantic Current at the site location during the mid-Pleistocene transition, between Marine Isotope Stage (MIS) 35 and 15.Three intervals have been described based on changes in nannofossil composition and abundance fluctuations of taxa. Reticulofenestrids are abundant just in the first interval (MIS 35–22), which is also characterised by the absence of Gephyrocapsa (> 4 μm) and C. pelagicus s.l., the latter occurring in upper MIS 24 only. The second interval corresponds to the expanded warm MIS 21 that is accompanied by high carbonate content, absence of both nannofossil reworking and ice rafted debris (IRD), higher abundance of C. leptoporus (5–8 μm) and H. carteri, and absence of C. pelagicus s.l. and medium gephyrocapsids. The third interval (MIS 20–15) records the occurrence of C. pelagicus s.l. and medium Gephyrocapsa, generally higher abundance and higher amplitude fluctuations of helicoliths, and absence of reticulofenestrids. The major changes in nannofossil assemblages seem to be the result of combined paleoenvironmental factors related to sea surface water features (temperature and nutrient availability, sea-ice cover) and possibly to deep and intermediate water production of both Northern and Southern Hemisphere origin during glacial and interglacial phases. Complex inter-relationships between environmental parameters and evolutionary/ecological factors are inferred to explain the patterns of Coccolithus pelagicus subspecies, whose distribution and abundance fluctuations are compared with the available patterns of Turborotalia quinqueloba and Neogloboquadrina pachyderma (left coiled) at Site 980 and with the C. pelagicus subspecies patterns from North Atlantic DSDP (Deep Sea Drilling Project) Site 607 and South Atlantic ODP Site 1090.Data collected provide additional information on stratigraphic distribution of selected taxa; the First Occurrence (FO) of Gephyrocapsa omega is confirmed as a diachronous event, even in the Atlantic Ocean, while the First and Last Common Occurrence (FCO, LCO) of R. asanoi appear as diachronous and synchronous events, respectively, on a global scale.