The Tropical Atlantic Response to Abrupt Climate Change during Interstadial 12
Hertzberg, Jennifer Eve
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The critical gaps in our understanding of the Earth's climate system lie in the patterns and forcing mechanisms of climate variability on decadal to submillennial time scales. Equally important and also not clearly understood is the response of low-latitude climate variability to abrupt climate change. most studies of abrupt change come from high-latitude sites. Dansgaard-Oeschger events such as Interstadial 12 are excellent examples of abrupt climate change, and provide an ideal opportunity to examine how the tropics and submillennial-scale variability respond to rapid changes in the global climate system. The Cariaco Basin's (Venezuela) high sedimentation rates, abundant microfossils, and excellent preservation make the site a superb location for high-resolution tropical Atlantic climate reconstructions. This study will present subdecadally resolved data for the critical intervals spanning the abrupt onset of Interstadial 12 (45,810. 44,668 yBP), and the transition back to stadial conditions (43,895. 42,831 yBP). Continuous sampling of a Calypso-style piston core at 1 mm intervals and dated with a novel sediment element concentration-based approach resulted in an average sample resolution of ~ 2 years, making this the highest-resolved record of an interstadial event to date. Foraminiferal census data were collected on samples for the onset and transition out of Interstadial 12, and were interpreted in the context of modern ecological preferences and the forcing mechanisms responsible for apparent patterns. The interpretation of the abundance records for the onset of Interstadial 12 is complicated by the combined and competing effects of rising sea level on Ekman-induced upwelling within the Cariaco Basin and migrating Intertropical Convergence Zone (ITCZ)-associated variations in trade wind location and fluvial nutrient delivery to the basin. However, the results suggest a northerly shift in the average annual position of the ITCZ by 600. 700 km in as little as a few centuries, with most of the shift occurring in less than 75 years. The foraminiferal abundance records for the latter part of the interstadial suggest a southerly shift in the average annual position of the ITCZ that acted to enhance upwelling and productivity within the Cariaco Basin. Sea level eventually reached a critical point in the transition back to stadial conditions that led to upwelling of nutrient-depleted waters and a decline in productivity within the basin. The most probable potential forcing mechanism of a north/south migration of the ITCZ during the interstadial/stadial periods of the last glacial period is likely related to the surface expression of variation in Atlantic meridional overturning circulation. Spectral and wavelet analyses of the Globigerina bulloides absolute abundance records for the onset and termination of Interstadial 12 were performed to isolate dominant periodicities in the data and identify possible forcing mechanisms and related linkages. The results reveal significant variability ranging from interannual to multicentennial scale, some of which are likely related to equatorial Atlantic Ocean dynamics, important air-sea interactions, and potential teleconnections. The influence of North Atlantic Oscillation-type climate variability may have extended further south during the cold stadials of the last glacial period. Atlantic Multidecadal Oscillation (AMO)-type climate variability is only evident in the warmest interval of Interstadial 12, suggesting that AMO-type variability may only operate during warm climate periods, something that has significant implications for modern and near-future climate variability.