Natalie L Roberts

Synchronous Deglacial Overturning and Water Mass Source Changes

The Depths of the Changes Over the course of the past glacial cycle, there have been two major types of rapid, large climate warming events: shorter-lived warm intervals lasting on the order of 1000 years and the last glacial-interglacial transition. Although both involved dramatic changes in large-scale ocean circulation, the extent to which those changes were similar is unclear. Roberts et al. (p. 75) analyzed the neodymium isotopic composition of the Fe-Mn oxide coatings of planktonic foraminifera and reconstructed patterns of Atlantic Ocean circulation during Heinrich event 1, a rapid global climate fluctuation about 14,000 years ago involving the destruction of Northern Hemisphere ice shelves and the last deglaciation. While both the source of deep water and the whole-ocean overturning rate shifted rapidly and synchronously during the last deglacial transition, only upper ocean circulation strength was affected during Heinrich event 1. Large-scale ocean circulation changed in different ways during a millennial-scale climate event. Understanding changes in ocean circulation during the last deglaciation is crucial to unraveling the dynamics of glacial-interglacial and millennial climate shifts. We used neodymium isotope measurements on postdepositional iron-manganese oxide coatings precipitated on planktonic foraminifera to reconstruct changes in the bottom water source of the deep western North Atlantic at the Bermuda Rise. Comparison of our deep water source record with overturning strength proxies shows that both the deep water mass source and the overturning rate shifted rapidly and synchronously during the last deglacial transition. In contrast, any freshwater perturbation caused by Heinrich event 1 could have only affected shallow overturning. These findings show how changes in upper-ocean overturning associated with millennial-scale events differ from those associated with whole-ocean deglacial climate events.

North Atlantic ocean circulation and abrupt climate change during the last glaciation

An ocean of climate impacts Large decreases in Atlantic meridional overturning circulation accompanied every one of the cold Northern Hemispheric stadial events that occurred during the heart of the last glacial period. These events, lasting on average around 1000 years each, have long been thought to result from changes in deep ocean circulation. Henry et al. used a suite of geochemical proxies from marine sediments to show that reductions in the export of northern deep waters occurred before and during stadial periods (see the Perspective by Schmittner). This observation firmly establishes the role of ocean circulation as a cause of abrupt glacial climate change during that interval. Science, this issue p. 470; see also p. 445 Deep ocean circulation changes preceded and accompanied the millennial cold climate events of the past ice age. The most recent ice age was characterized by rapid and hemispherically asynchronous climate oscillations, whose origin remains unresolved. Variations in oceanic meridional heat transport may contribute to these repeated climate changes, which were most pronounced during marine isotope stage 3, the glacial interval 25 thousand to 60 thousand years ago. We examined climate and ocean circulation proxies throughout this interval at high resolution in a deep North Atlantic sediment core, combining the kinematic tracer protactinium/thorium (Pa/Th) with the deep water-mass tracer, epibenthic δ13C. These indicators suggest reduced Atlantic overturning circulation during every cool northern stadial, with the greatest reductions during episodic Hudson Strait iceberg discharges, while sharp northern warming followed reinvigorated overturning. These results provide direct evidence for the ocean’s persistent, central role in abrupt glacial climate change.

Evidence for elevated alkalinity in the glacial Southern Ocean

An increase in whole ocean alkalinity during glacial periods could account, in part, for the drawdown of atmospheric CO2 into the ocean. Such an increase was inevitable due to the near elimination of shelf area for the burial of coral reef alkalinity. We present evidence, based on downcore measurements of benthic foraminiferal B/Ca and Mg/Ca from a core in the Weddell Sea, that the deep ocean carbonate ion concentration, [CO32-], was elevated by similar to 25 mu mol/kg during each glacial period of the last 800 kyr. The heterogeneity of the preservation histories in the different ocean basins reflects control of the carbonate chemistry of the deep glacial ocean in the Atlantic and Pacific by the changing ventilation and chemistry of Weddell Sea waters. These waters are more corrosive than interglacial northern sourced waters but not as undersaturated as interglacial southern sourced waters. Our inferred increase in whole ocean alkalinity can be reconciled with reconstructions of glacial saturation horizon depth and the carbonate budget if carbonate burial rates also increased above the saturation horizon as a result of enhanced pelagic calcification. The Weddell records display low [CO32-] during deglaciations and peak interglacial warmth, coincident with maxima in percent CaCO3 in the Atlantic and Pacific oceans. Should the burial rate of alkalinity in the more alkaline glacial deep waters outstrip the rate of alkalinity supply, then pelagic carbonate production by the coccolithophores at the end of the glacial maximum could drive a decrease in ocean [CO32-] and act to trigger the deglacial rise in pCO(2).

Advection and scavenging controls of Pa/Th in the northern NE Atlantic

Over the last 2 decades, significant advances have been made in reconstructing past rates of ocean circulation using sedimentary proxies for the dynamics of abyssal waters. In this study we combine the use of two rate proxies, sortable silt grain size, and sedimentary 231Pa/230Th, measured on a depth transect of deep-sea sediment cores from the northern NE Atlantic, to investigate ocean circulation changes during the last deglacial. We find that at two deep sites, the core-top 231Pa/230Th ratios reflect Holocene circulation rates, while during Heinrich Stadial 1, the deglacial ratios peaked as the sortable silt grain size decreased, reflecting a general circulation slowdown. However, the peak 231Pa/230Th significantly exceeded the production ratio in both cores, indicating that 231Pa/230Th was only partially controlled by ocean circulation at these sites. This is supported by a record of 231Pa/230Th from an intermediate water depth site, where values also peaked during Heinrich Stadial 1, but were consistently above the production ratio over the last 24 ka, reflecting high scavenging below productive surface waters. At our study sites, we find that preserved sediment component fluxes cannot be used to distinguish between a scavenging or circulation control, although they are consistent with a circulation influence, since the core at intermediate depth with the highest 231Pa/230Th recorded the lowest particle fluxes. Reconstruction of advection rate using 231Pa/230Th in this region is complicated by high productivity, but the data nevertheless contain important information on past deep ocean circulation.