International audienceThe strength and geometry of the Atlantic meridional overturning circulation is tightly coupled to climate on glacial-interglacial and millennial timescales(1), but has proved difficult to reconstruct, particularly for the Last Glacial Maximum(2). Today, the return flow from the northern North Atlantic to lower latitudes associated with the Atlantic meridional overturning circulation reaches down to approximately 4,000 m. In contrast, during the Last Glacial Maximum this return flow is thought to have occurred primarily at shallower depths. Measurements of sedimentary Pa-231/Th-230 have been used to reconstruct the strength of circulation in the North Atlantic Ocean(3,4), but the effects of biogenic silica on Pa-231/Th-230-based estimates remain controversial(5). Here we use measurements of Pa-231/Th-230 ratios and biogenic silica in Holocene-aged Atlantic sediments and simulations with a two-dimensional scavenging model to demonstrate that the geometry and strength of the Atlantic meridional overturning circulation are the primary controls of Pa-231/Th-230 ratios in modern Atlantic sediments. For the glacial maximum, a simulation of Atlantic overturning with a shallow, but vigorous circulation and bulk water transport at around 2,000 m depth best matched observed glacial Atlantic Pa-231/Th-230 values. We estimate that the transport of intermediate water during the Last Glacial Maximum was at least as strong as deep water transport today
During the last glacial period, Greenland's climate shifted between cold (stadial) and warm (interstadial) phases that were accompanied by ocean circulation changes characterized by reduced Atlantic Meridional Overturning Circulation (AMOC) during stadials. Here we present new data from the western tropical Atlantic demonstrating that AMOC slowdowns preceded some of the large South American rainfall events that took place during stadials. Based on 231 Pa/ 230 Th and Ti/Ca measurements in the same sediment core, we determine that the AMOC started to slowdown 1420 ± 250 and 690 ± 180 (1σ) years before the onset of two large precipitation events associated with Heinrich stadials. Our results bring unprecedented evidence that AMOC changes could be at the origin of the large precipitation events observed in tropical South America during Heinrich stadials. In addition, we propose a mechanism explaining the differences in the extent and timing of AMOC slowdowns associated with shorter and longer stadials.
Abstract. Thanks to its optimal location on the northern Brazilian margin,
core MD09-3257 records both ocean circulation and atmospheric changes. The
latter occur locally in the form of increased rainfall on the adjacent
continent during the cold intervals recorded in Greenland ice and northern
North Atlantic sediment cores (i.e., Greenland stadials). These rainfall
events are recorded in MD09-3257 as peaks in ln(Ti ∕ Ca). New sedimentary
Pa ∕ Th data indicate that mid-depth western equatorial water mass
transport decreased during all of the Greenland stadials of the last 40 kyr.
Using cross-wavelet transforms and spectrogram analysis, we assess the
relative phase between the MD09-3257 sedimentary Pa ∕ Th and
ln(Ti ∕ Ca) signals. We show that decreased water mass transport between
a depth of ∼1300 and 2300 m in the western equatorial Atlantic preceded
increased rainfall over the adjacent continent by 120 to 400 yr at
Dansgaard–Oeschger (D–O) frequencies, and by 280 to 980 yr at Heinrich-like
frequencies. We suggest that the large lead of ocean circulation changes with respect to
changes in tropical South American precipitation at Heinrich-like
frequencies is related to the effect of a positive feedback involving
iceberg discharges in the North Atlantic. In contrast, the absence of
widespread ice rafted detrital layers in North Atlantic cores during D–O
stadials supports the hypothesis that a feedback such as this was not triggered in
the case of D–O stadials, with circulation slowdowns and subsequent changes
remaining more limited during D–O stadials than Heinrich stadials.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.