Variations in the Atlantic Meridional Overturning Circulation (AMOC) redistribute heat and nutrients, causing pronounced anomalies of temperature and nutrient concentrations in the subsurface ocean. However, exactly how millennial‐scale deglacial AMOC variability influenced the subsurface is debated, and the role of other deglacial forcings of subsurface temperature change is unclear. Here, we present a new deglacial temperature reconstruction, which, with published records, helps assess competing hypotheses for deglacial warming in the upper tropical North Atlantic. Our record provides new evidence of regional subsurface warming in the western tropical North Atlantic within the core of modern Antarctic Intermediate Water (AAIW) during Heinrich Stadial 1 (HS1), an early deglacial interval of iceberg discharge into the North Atlantic. Our results are consistent with model simulations that suggest subsurface heat accumulates in the northern high‐latitude convection regions and along the upper AMOC return path when the AMOC weakens, and with warming due to rising greenhouse gases. Warming of AAIW may have also contributed to warming in the tropics at modern AAIW depths during late HS1. Nutrient and
Atlantic Meridional Overturning Circulation (AMOC) disruption during the last deglaciation is hypothesized to have caused large subsurface ocean temperature anomalies, but records from key regions are not available to test this hypothesis, and other possible drivers of warming have not been fully considered. Here, we present the first reliable evidence for subsurface warming in the South Atlantic during Heinrich Stadial 1, confirming the link between large‐scale heat redistribution and AMOC. Warming extends across the Bølling‐Allerød despite predicted cooling at this time, thus spanning intervals of both weak and strong AMOC indicating another forcing mechanism that may have been previously overlooked. Transient model simulations and quasi‐conservative water mass tracers suggest that reduced northward upper ocean heat transport was responsible for the early deglacial (Heinrich Stadial 1) accumulation of heat at our shallower (~1,100 m) site. In contrast, the results suggest that warming at our deeper site (~1,900 m) site was dominated by southward advection of North Atlantic middepth heat anomalies. During the Bølling‐Allerød, the demise of ice sheets resulted in oceanographic changes in the North Atlantic that reduced convective heat loss to the atmosphere, causing subsurface warming that overwhelmed the cooling expected from an AMOC reinvigoration. The data and simulations suggest that rising atmospheric CO2did not contribute significantly to deglacial subsurface warming at our sites.
more » « less- NSF-PAR ID:
- 10460719
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Paleoceanography and Paleoclimatology
- Volume:
- 34
- Issue:
- 6
- ISSN:
- 2572-4517
- Page Range / eLocation ID:
- p. 990-1005
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
Abstract reconstructions from the same site suggest a link between AMOC intensity and the northward extent of AAIW in the northern tropics across the deglaciation and on millennial time scales. However, the timing of the initial deglacial increase in AAIW to the northern tropics is ambiguous. Deglacial trends and variability of in the upper North Atlantic have likely biased temperature reconstructions based on the elemental composition of calcitic benthic foraminifera. -
more » « less
Abstract The phase relationships of sea surface temperature (SST) changes between the North Pacific and North Atlantic during deglacial millennial‐scale climate events have been of great interest. However, uncertainties remain partly due to the sparsity of deglacial SST records in the North Pacific. This study presents a new high‐resolution
‐SST record spanning the entire last deglaciation from core LV63‐41‐2 retrieved from the Northwestern Pacific off the Kamchatka Peninsula, which allows us to explore regional SST change patterns and associated driving mechanisms by compiling previously published SST data in the subarctic Pacific. The subarctic Pacific SST changes during the Bølling‐Allerød and Younger Dryas show in‐phase relationships in response to the North Atlantic SST variations, suggesting a dominant control of atmospheric teleconnections between both oceans. During Heinrich Stadial 1 (HS1) when the North Atlantic exhibited significant cooling, the subarctic Pacific SST developments are complex, showing gradual warming from the Last Glacial Maximum to HS1 in the Northwestern Pacific and cooling at the onset of HS1 in the Northeastern Pacific. We suggest that the inconsistent phase responses resulted from the combined effects of multiple processes, which involve an enhanced poleward advection of warm subtropical waters, cold meltwater inputs from the retreating Cordilleran Ice Sheet into the Northeastern Pacific, and a persistent La Niña‐like state in the tropical Pacific. -
more » « less
Abstract Heinrich Stadial 1 (HS1) was the major climate event at the onset of the last deglaciation associated with rapid cooling in Greenland and lagged, slow warming in Antarctica. Although it is widely believed that temperature signals were triggered in the Northern Hemisphere and propagated southward associated with the Atlantic meridional overturning circulation (AMOC), understanding how these signals were able to cross the Antarctic Circumpolar Current (ACC) barrier and further warm up Antarctica has proven particularly challenging. In this study, we explore the physical processes that lead to the Antarctic warming during HS1 in a transient isotope-enabled deglacial simulation iTRACE, in which the interpolar phasing has been faithfully reproduced. We show that the increased meridional heat transport alone, first through the ocean and then through the atmosphere, can explain the Antarctic warming during the early stage of HS1 without notable changes in the strength and position of the Southern Hemisphere midlatitude westerlies. In particular, when a reduction of the AMOC causes ocean warming to the north of the ACC, increased southward ocean heat transport by mesoscale eddies is triggered by steeper isopycnals to warm up the ocean beyond the ACC, which further decreases the sea ice concentration and leads to more absorption of insolation. The increased atmospheric heat then releases to the Antarctic primarily by a strengthening zonal wavenumber-3 (ZW3) pattern. Sensitivity experiments further suggest that a ∼4°C warming caused by this mechanism superimposed on a comparable warming driven by the background atmospheric CO2rise is able to explain the total simulated ∼8°C warming in the West Antarctica during HS1.
-
more » « less
Abstract The last deglaciation (~20–10 kyr BP) was characterized by a major shift in Earth's climate state, when the global mean surface temperature rose ~4 °C and the concentration of atmospheric CO2increased ~80 ppmv. Model simulations suggest that the initial 30 ppmv rise in atmospheric CO2may have been driven by reduced efficiency of the biological pump or enhanced upwelling of carbon‐rich waters from the abyssal ocean. Here we evaluate these hypotheses using benthic foraminiferal B/Ca (a proxy for deep water [CO32−]) from a core collected at 1,100‐m water depth in the Southwest Atlantic. Our results imply that [CO32−] increased by 22 ± 2 μmol/kg early in Heinrich Stadial 1, or a decrease in ΣCO2of approximately 40 μmol/kg, assuming there were no significant changes in alkalinity. Our data imply that remineralized phosphate declined by approximately 0.3 μmol/kg during Heinrich Stadial 1, equivalent to 40% of the modern remineralized signal at this location. Because tracer inversion results indicate remineralized phosphate at the core site reflects the integrated effect of export production in the sub‐Antarctic, our results imply that biological productivity in the Atlantic sector of the Southern Ocean was reduced early in the deglaciation, contributing to the initial rise in atmospheric CO2.
