Southern Exposure: New Paleoclimate Insights From Southern Ocean and Antarctic Margin Sediments

Much of what is known about the evolution of Antarctica's cryosphere in the geologic past is derived from ice-distal deep-sea sedimentary records. Recent advances in drilling technology and climate proxy methods have made it possible to retrieve and interpret high-quality ice-proximal sedimentary sequences from Antarctica's margins and the Southern Ocean. These records contain a wealth of information about the individual histories of the East and West Antarctic Ice Sheets and associated temperature change in the circum-Antarctic seas. Emerging studies of Antarctic drill cores provide evidence of dynamic climate variability on both short and long timescales over the past 20 million years. This geologic information is critical for testing and improving computer model simulations used to predict future environmental change in the polar regions. Identifying the mechanistic links between past Antarctic ice-volume fluctuations and oceanographic change is necessary for understanding Earth's long-term climate evolution. While recent successes highlight the value of ice-proximal records, additional scientific drilling and climate proxy development are required to improve current knowledge of Antarctica's complex paleoenvironmental history

North Atlantic marine biogenic silica accumulation through the early-to-mid Paleogene: implications for ocean circulation and silicate weathering feedback

The Paleogene history of biogenic opal accumulation in the North Atlantic provides insight into both the evolution of deep-water circulation in the Atlantic basin, and weathering responses to major climate shifts. However, existing records are compromised by low temporal resolution and/or stratigraphic discontinuities. In order to address this problem, we present a multi-site, high-resolution record of biogenic silica (bioSiO2) accumulation from Blake Nose (ODP Leg 171B, western North Atlantic) spanning the early Paleocene through late Eocene time interval (~65‒34 Ma). This record represents the longest single-locality history of marine bioSiO2 burial compiled to date and offers a unique perspective into changes in bioSiO2 fluxes through the early-to-mid Paleogene extreme greenhouse interval and subsequent period of long-term cooling. Blake Nose bioSiO2 fluxes display prominent fluctuations that we attribute to variations in sub-thermocline nutrient supply via cyclonic eddies associated with the Gulf Stream. Whereas few constraints are available on the bioSiO2 flux pulses peaking in the early Paleocene and early Eocene, a middle Eocene interval of elevated bioSiO2 flux between ~46 and 42 Ma is proposed to reflect nutrient enrichment due to invigorated overturning circulation following an early onset of Northern Component Water export from the Norwegian-Greenland Sea at ~49 Ma. Comparison of our North Atlantic record against published Pacific bioSiO2 flux records indicates a diminished nutrient supply to the Atlantic between ~42 and 38 Ma, interpreted as a response to weakening of the overturning circulation. Subsequently, a deep-water circulation regime favoring limited bioSiO2 burial in Atlantic and enhanced bioSiO2 burial in the Pacific was established after ~38 Ma, likely in association with a further invigoration of deep-water export from the North Atlantic. We also observe that Blake Nose bioSiO2 fluxes through the middle Eocene cooling interval (~48 to 34 Ma) are consistently higher than background fluxes throughout the late Paleocene‒early Eocene interval of intense greenhouse warmth. This observation is consistent with a temporally variable rather than constant silicate weathering feedback strength model for the Paleogene, which would instead predict that marine bioSiO2 burial should peak during periods of extreme warming

The early to middle Eocene transition: an integrated calcareous nannofossil and stable isotope record from the Northwest Atlantic Ocean (IODP Site U1410)

The early to middle Eocene is marked by prominent changes in calcareous nannofossil assemblages coinciding both with long‐term climate changes and modification of the North Atlantic deep‐ocean circulation. In order to assess the impact of Eocene climate change on surface‐water environmental conditions of the Northwest Atlantic, we developed calcareous nannoplankton assemblage data and bulk stable isotope records (δ18O and δ13C) across an early to middle Eocene interval (~52–43 Ma) at IODP Site U1410 (Southeast Newfoundland Ridge, ~41°N). At this site, early Eocene sediments are pelagic nannofossil chalk, whereas middle Eocene deposits occur as clay‐rich drift sediments reflecting the progressive influence of northern‐sourced deep currents. Between the end of Early Eocene Climatic Optimum and the Ypresian/Lutetian boundary, calcareous nannofossils switched from an assemblage mainly composed of warm‐water and oligotrophic taxa (Zygrhablithus, Discoaster, Sphenolithus, Coccolithus) to one dominated by the more temperate and eutrophic reticulofenestrids. The most prominent period of accelerated assemblage change occurred during a ~2 Myr phase of relatively high bulk δ18O values possibly related to the post‐EECO cooling. Although the dominance of reticulofenestrids persisted unvaried throughout the middle Eocene interval, early Lutetian (~47.4 to 47 Ma) stable isotope records indicate a reversal in the paleoenvironmetal trends suggesting a potential restoration of warmer conditions. Importantly, our data indicate that the ~2 Myr‐interval immediately following the EECO was crucial in establishing the modern calcareous nannofossil assemblage structure and also reveal that the establishment of Reticulofenestra‐dominated assemblage occurred prior to the onset of persistent deep‐current system in the Northwest Atlantic