Expanded oxygen minimum zones during the late Paleocene-early Eocene:Hints from multiproxy comparison and ocean modeling

Anthropogenic warming could well drive depletion of oceanic oxygen in the future. Important insight into the relationship between de-oxygenation and warming can be gleaned from the geological record, but evidence is limited because few ocean oxygenation records are available for past greenhouse climate conditions. We use I/Ca in benthic foraminifera to reconstruct late Paleocene through early Eocene bottom and pore-water redox conditions in the South Atlantic and Southern Indian Oceans, and compare our results with those derived from Mn speciation and the Ce anomaly in fish teeth. We conclude that waters with lower oxygen concentrations were widespread at intermediate depths (1.5-2 km), whereas bottom waters were more oxygenated at the deepest site, in the Southeast Atlantic Ocean (>3 km). Epifaunal benthic foraminiferal I/Ca values were higher in the late Paleocene, especially at low oxygen sites, than at well-oxygenated modern sites, indicate higher seawater total iodine concentrations in the late Paleocene than today. The proxy-based bottom water oxygenation pattern agrees with the site-to-site O2 gradient as simulated in a comprehensive climate model (CCSM3), but the simulated absolute dissolved O2 values are low (<~35 µmol/kg), while higher O2 values (~60-100 µmol/kg) were obtained in an Earth system model (cGENIE). Multi-proxy data together with improvements in boundary conditions and model parameterization are necessary if the details of past oceanographic oxygenation are to be resolved

Biogenic sedimentation in the Eocene equatorial Pacific: the stuttering greenhouse and Eocene carbonate compensation depth

CaCO3, Corg, and biogenic SiO2 were measured in Eocene equatorial Pacific sediments from Sites 1218 and 1219, and bulk oxygen and carbon isotopes were measured on selected intervals from Site 1219. These data delineate a series of CaCO3 events that first appeared at ~48 Ma and continued to the Eocene/Oligocene boundary. Each event lasted 1–2 m.y. and is separated from the next by a low CaCO3 interval of a similar time span. The largest of these carbonate accumulation events (CAE-3) is in Magnetochron 18. It began at ~42.2 Ma, lasted until ~40.3 Ma, and was marked by higher than average productivity. The end of CAE-3 was abrupt and was associated with a large-scale carbon transfer to the oceans prior to warming of high-latitude regions. Changes in carbonate compensation depth associated with CAE excursions were small in the early part of the middle Eocene but increased to as much as 800 m by the late middle Eocene before decreasing into the late Eocene. Oxygen isotope data indicate that the carbonate events are associated with cooling conditions and may mark small glaciations in the Eocene

Millennial-Scale CaCO\u3csub\u3e3\u3c/sub\u3e and C\u3csub\u3eorg\u3c/sub\u3e Events Along the Northern and Central California Margin

Sediments from five Leg 167 drill sites and three piston cores were analyzed for Corg and CaCO3. Oxygen isotope stratigraphy on benthic foraminifers was used to assign age models to these sedimentary records. We find that the northern and central California margin is characterized by k.y.-scale events that can be found in both the CaCO3 and Corg time series. We show that the CaCO3 events are caused by changes in CaCO3 production by plankton, not by dissolution. We also show that these CaCO3 events occur in marine isotope Stages (MIS) 2, 3, and 4 during Dansgaard/Oeschger interstadials. They occur most strongly, however, on the MIS 5/4 glaciation and MIS 2/1 deglaciation. We believe that the link between the northeastern Pacific Ocean and North Atlantic is primarily transmitted by the atmosphere, not the ocean. Highest CaCO3 production and burial occurs when the surface ocean is somewhat cooler than the modern ocean, and the surface mixed layer is somewhat more stable

Data–model integration for the Miocene: the need for more ocean drilling

It is clear that the Miocene is a key interval to understand how earth systems interact to maintain global warmth, especially since carbon-based greenhouse gases may not have been extremely elevated at that time. It is also much easier, potentially, to study the Miocene than earlier intervals of global warmth because more complete sedimentary sections exist. To be studied, however, the sediment sections need to be sampled. In the oceans, the Miocene is sampled by the Integrated Ocean Drilling Program and its predecessors (Ocean Drilling Program and Deep Sea Drilling Project).Marine sedimentary records need to be gathered from the oceans at a scale to resolve the temperature evolution of the major surface water masses and transition regions. New drilling targeted to study the Miocene must occur to achieve this distribution. With existing ODP and DSDP cores it is possible to sample and study general trends over the Miocene, but it is only possible to study the early and middle Miocene in high resolution for a few ocean areas. Pelagic Miocene sections as a rule tend to be buried deeply below the surface and have not been cored adequately to make composite sediment columns needed for detailed studies.In the Pacific, for example, there is relatively good coverage to the middle-late Miocene boundary (~12Ma) but continuous sedimentary sections that cover the early and middle Miocene are relatively rare. Ironically, there are relatively good new data from the Pacific subtropical gyres, but information from the equatorial region and the high latitudes is lacking. A new site survey (AMAT-03) and a scheduled IODP drilling program have produced a strategy to measure a continuous 50 million year record of the equatorial Pacific Ocean. Similar programs are needed for other representative regions of the ocean and ocean gateways

Millennial–scale CaCO₃ and C[subscript ORG] events along the northern and central California margins : stratigraphy and origins

Sediments from five Leg 167 drill sites and three piston cores were analyzed for C[subscript ORG] and CaCO₃. Oxygen isotope stratigraphy on benthic foraminifers was used to assign age models to these sedimentary records. We find that the northern and central California margin is characterized by k.y.-scale events that can be found in both the CaCO₃ and C[subscript ORG] time series. We show that the CaCO₃ events are caused by changes in CaCO₃ production by plankton, not by dissolution. We also show that these CaCO₃ events occur in marine isotope Stages (MIS) 2, 3, and 4 during Dansgaard/Oeschger interstadials. They occur most strongly, however, on the MIS 5/4 glaciation and MIS 2/1 deglaciation. We believe that the link between the northeastern Pacific Ocean and North Atlantic is primarily transmitted by the atmosphere, not the ocean. Highest CaCO₃ production and burial occurs when the surface ocean is somewhat cooler than the modern ocean, and the surface mixed layer is somewhat more stable

Warm ocean processes and carbon cycling in the Eocene

Sea surface and subsurface temperatures over large parts of the ocean during the Eocene epoch (55.5–33.7?Ma) exceeded modern values by several degrees, which must have affected a number of oceanic processes. Here, we focus on the effect of elevated water column temperatures on the efficiency of the biological pump, particularly in relation to carbon and nutrient cycling. We use stable isotope values from exceptionally well-preserved planktonic foraminiferal calcite from Tanzania and Mexico to reconstruct vertical carbon isotope gradients in the upper water column, exploiting the fact that individual species lived and calcified at different depths. The oxygen isotope ratios of different species' tests are used to estimate the temperature of calcification, which we converted to absolute depths using Eocene temperature profiles generated by general circulation models. This approach, along with potential pitfalls, is illustrated using data from modern core-top assemblages from the same area. Our results indicate that, during the Early and Middle Eocene, carbon isotope gradients were steeper (and larger) through the upper thermocline than in the modern ocean. This is consistent with a shallower average depth of organic matter remineralization and supports previously proposed hypotheses that invoke high metabolic rates in a warm Eocene ocean, leading to more efficient recycling of organic matter and reduced burial rates of organic carbon