Seawater Cadmium in the Florida Straits Over the Holocene and Implications for Upper AMOC Variability

Abstract

Atlantic Meridional Overturning Circulation (AMOC) plays a central role in the global redistribution of heat and precipitation during both abrupt and longer‐term climate shifts. Over the next century, AMOC is projected to weaken due to greenhouse gas warming, though projecting its future behavior is dependent on a better understanding of how AMOC changes are forced. Seeking to resolve an apparent contradiction of AMOC trends from paleorecords of the more recent past, we reconstruct seawater cadmium, a nutrient‐like tracer, in the Florida Straits over the last ∼8,000 years, with emphasis on the last millennium. The gradual reduction in seawater Cd over the last 8,000 years could be due to a reduction in AMOC, consistent with cooling Northern Hemisphere temperatures and a southward shift of the Intertropical Convergence Zone. However, it is difficult to reconcile this finding with evidence for an increase in geostrophic flow through the Florida Straits over the same time period. We combine data from intermediate water depth sediment cores to extend this record into the Common Era at sufficient resolution to address the broad scale changes of this time period. There is a small decline in the Cd concentration in the Late Little Ice Age relative to the Medieval Climate Anomaly, but this change was much smaller than the changes observed over the Holocene and on the deglaciation. This suggests that any trend in the strength of AMOC over the last millennium must have been very subtle.

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

“…Tropical CdW values diverge from subtropical North Atlantic values (Came et al, 2008;Valley et al, 2017) and subpolar North Atlantic values (Bertram et al, 1995;Yu & Elderfield, 2007;Yu et al, 2019) (Figure S9 in Supporting Information S1), also consistent with increasing AAIW relative to northern source waters at the sites, and an AMOC strengthening. Although a Holocene decrease in CdW at the Florida Margin has been interpreted to indicate a weakening of the AMOC (Valley et al, 2022), much of the CdW decrease at the Florida Margin occurred after ∼5 ka, so we do not rule out an early Holocene AMOC strengthening, followed by a later Holocene weakening. values at ∼9 ka.…”

“…(2019), and Valley et al. (2017, 2019, 2022). Data generated for this study are available in Oppo et al.…”

“…Figure 3. Relationship between bottom water temperature (BWT) and core-top H. elegans Mg/Li data generated at INSTAAR (red)(Bryan & Marchitto, 2008;Marchitto et al, 2018;Valley et al, 2019Valley et al, , 2022 and Woods Hole Oceanographic Institution (Demerara Rise in blue, other sites in cyan; this study,Umling et al, 2019) (Data Set S3 in Supporting Information S1). Solid black line indicates calibration fromMarchitto et al (2018).…”

No Consistent Simulated Trends in the Atlantic Meridional Overturning Circulation for the Past 6,000 Years

“…Although the IPCC assessment of reconstructions suggest little change in AMOC during most of the past 6,000 years (Gulev et al, 2021), there are several proxy reconstructions that posit a late Holocene decline (Ayache et al, 2018;Valley et al, 2022). It may be that these reflect changes in individual components of the AMOC, such as Iceland-Scotland overflow water.…”

“…Data assimilation approaches similarly suggest little long-term AMOC change in the Holocene (Osman et al, 2021;Ritz et al, 2013). Higher temporal resolution reconstructions for the past 2,000 years also show little change in overall AMOC, but exhibit uncertainty about the timing and nature of the Industrial Era decline (Caesar et al, 2021;Thornalley et al, 2018;Valley et al, 2022). Reconstructions exist that propose different temporal behavior; combining all the existing records to provide a complete picture of all aspects of the AMOC remains an ongoing effort.…”