Chandranath Basak

Nutrient trap for Late Cretaceous organic-rich black shales in the tropical North Atlantic

Neodymium isotopes of fish debris from two sites on Demerara Rise, spanning ∼4.5 m.y. of deposition from the early Cenomanian to just before ocean anoxic event 2 (OAE2) (Cenomanian-Turonian transition), suggest a circulation-controlled nutrient trap in intermediate waters of the western tropical North Atlantic that could explain continuous deposition of organic-rich black shales for as many as ∼15 m.y. (Cenomanian–early Santonian). Unusually low Nd isotopic data (e Nd(t) ∼−11 to ∼−16) on Demerara Rise during the Cenomanian are confirmed, but the shallower site generally exhibits higher and more variable values. A scenario in which southwest-flowing Tethyan and/or North Atlantic waters overrode warm, saline Demerara bottom water explains the isotopic differences between sites and could create a dynamic nutrient trap controlled by circulation patterns in the absence of topographic barriers. Nutrient trapping, in turn, would explain the ∼15 m.y. deposition of black shales through positive feedbacks between low oxygen and nutrient-rich bottom waters, efficient phosphate recycling, transport of nutrients to the surface, high productivity, and organic carbon export to the seafloor. This nutrient trap and the correlation seen previously between high Nd and organic carbon isotopic values during OAE2 on Demerara Rise suggest that physical oceanographic changes could be components of OAE2, one of the largest perturbations to the global carbon cycle in the past 150 m.y.

Pliocene cooling enhanced by flow of low-salinity Bering Sea water to the Arctic Ocean

Warming of high northern latitudes in the Pliocene (5.33–2.58 Myr ago) has been linked to the closure of the Central American Seaway and intensification of North Atlantic Deep Water. Subsequent cooling in the late Pliocene may be related to the effects of freshwater input from the Arctic Ocean via the Bering Strait, disrupting North Atlantic Deep Water formation and enhancing sea ice formation. However, the timing of Arctic freshening has not been defined. Here we present neodymium and lead isotope records of detrital sediment from the Bering Sea for the past 4.3 million years. Isotopic data suggest the presence of Alaskan glaciers as far back as 4.2 Myr ago, while diatom and C37:4 alkenone records show a long-term trend towards colder and fresher water in the Bering Sea beginning with the M2 glaciation (3.3 Myr ago). We argue that the introduction of low-salinity Bering Sea water to the Arctic Ocean by 3.3 Myr ago preconditioned the climate system for global cooling.

Breakup of last glacial deep stratification in the South Pacific

CO2 escaped from the deep Why did the concentration of atmospheric carbon dioxide rise so much and so quickly during the last deglaciation? Evidence has begun to accumulate suggesting that old, carbon-rich water accumulated at depth in the Southern Ocean, which then released its charge when Southern Ocean stratification broke down as the climate there warmed. Basak et al. present measurements of neodymium isotopes that clearly show that the deepwater column of the glacial southern South Pacific was stratified, just as would be necessary for the accumulation of old, carbon-rich water. Their data also show that North Atlantic processes were not the dominant control on Southern Ocean water-mass structure during that interval, as has been thought. Science, this issue p. 900 The glacial southern South Pacific was strongly stratified at the end of the last glacial period. Stratification of the deep Southern Ocean during the Last Glacial Maximum is thought to have facilitated carbon storage and subsequent release during the deglaciation as stratification broke down, contributing to atmospheric CO2 rise. Here, we present neodymium isotope evidence from deep to abyssal waters in the South Pacific that confirms stratification of the deepwater column during the Last Glacial Maximum. The results indicate a glacial northward expansion of Ross Sea Bottom Water and a Southern Hemisphere climate trigger for the deglacial breakup of deep stratification. It highlights the important role of abyssal waters in sustaining a deep glacial carbon reservoir and Southern Hemisphere climate change as a prerequisite for the destabilization of the water column and hence the deglacial release of sequestered CO2 through upwelling.