Deglacial Temperature and Carbonate Saturation State Variability in the Tropical Atlantic at Antarctic Intermediate Water Depths

Abstract

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 Δ[CO32‐] 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 Δ[CO32‐] in the upper North Atlantic have likely biased temperature reconstructions based on the elemental composition of calcitic benthic foraminifera.

Arctic and Antarctic forcing of ocean interior warming during the last deglaciation

“…We find the magnitude of Equatorial Atlantic warming during HS1 is large (~ + 6 °C increase)—similar to records from the nearby Demerara Rise based on the Li/Mg ratio of benthic foraminifera 22 (Fig. 3 D).…”

“…However existing deglacial intermediate water records from the Atlantic do not agree on the timing and magnitude of temperature change. For instance, it is unclear whether warming commenced before 18 , 23 or after 16 , 17 , 22 the onset of HS1 at ~ 17 ka or the importance of warming during HS1 16 , 17 or the YD 18 , 23 (Fig. 3 D).…”

“…( C ) Previous East Equatorial Pacific intermediate water temperature records from benthic foraminifera δ 18 O 25 . ( D and E ) Examples of previous Equatorial and South Atlantic intermediate water temperature records from benthic foraminifera Mg/Ca and Li/Mg including: M78/1 235-1 (11° N, 852 m) 17 , KNR166-2-26JPC (24° N, 546 m) 23 , KNR197-3-46CDH (8° N, 947 m) 22 , KNR159‐5 ‐90GGC and -GGC36 (27° S, 1108 m and 1268 m) 16 , GC528 (53° S, 598 m) 19 . Coral Li/Mg intermediate water temperature reconstruction (this study) for ( F ) East Equatorial Pacific and Equatorial Atlantic, ( G ) Burdwood Bank (AAIW), ( H ) Cape Horn and ( I ) Sars/Interim (Upper Circumpolar Deep Water) sites.…”