Last Interglacial Hydroclimate Seasonality Reconstructed From Tropical Atlantic Corals

Abstract

The seasonality of hydroclimate during past periods of warmer than modern global temperatures is a critical component for understanding future climate change scenarios. Although only partially analogous to these scenarios, the last interglacial (LIG, Marine Isotope Stage 5e, ~127–117 ka) is a popular test bed. We present coral δ18O monthly resolved records from multiple Bonaire (southern Caribbean) fossil corals (Diploria strigosa) that date to between 130 and 118 ka. These records represent up to 37 years and cover a total of 105 years, offering insights into the seasonality and characteristics of LIG tropical Atlantic hydroclimate. Our coral δ18O records and available coral Sr/Ca‐sea surface temperature (SST) records reveal new insights into the variable relationship between the seasonality of tropical Atlantic seawater δ18O (δ18Oseawater) and SST. Coral δ18O seasonality is found to covary with SST and insolation seasonality throughout the LIG, culminating in significantly higher than modern values at 124 and 126 ka. At 124 ka, we reconstruct a 2 month lead of the coral δ18O versus the Sr/Ca‐SST annual cycle and increased δ18Oseawater seasonality. A fully coupled climate model simulates a concomitant increase of southern Caribbean Sea summer precipitation and depletion of summer δ18Oseawater. LIG hydroclimate at Bonaire differed from today's semiarid climate with a minor rainy season during winter. Cumulatively, our coral δ18O, δ18Oseawater, and model findings indicate a mid‐LIG northward expansion of the South American Intertropical Convergence Zone into the southern Caribbean Sea, highlighting the importance of regional aspects within model and proxy reconstructions of LIG hydroclimate seasonality.

Tropical Atlantic Cooling and Freshening in the Middle of the Last Interglacial From Coral Proxy Records

“…This tendency is similar to that observed during the middle to late Holocene (Giry et al, 2013). However, fresher than modern surface waters at 125.8 ka were not accompanied by seasonal indicators of hydroclimate such as the increased coral δ 18 O seawater seasonality (Figure 3c) and a two-month lead of coral δ 18 O versus Sr/Ca (Figure 3d) found at 123.9 ka by Brocas et al (2018a). Such a phase difference between paired proxy measurements that share a SST component and internal age model led to the conclusion that the annual cycle of surface waters peaked two months earlier than modern, occurring in July/August and December/January, respectively.…”

“…Such a phase difference between paired proxy measurements that share a SST component and internal age model led to the conclusion that the annual cycle of surface waters peaked two months earlier than modern, occurring in July/August and December/January, respectively. Furthermore, this coincided with higher than modern simulated tropical Atlantic precipitation (Brocas et al, 2018a;Nikolova et al, 2013;Pedersen et al, 2016) implying that at 123.9-ka precipitation dictated the hydrological regime in contrast to the modern oceanic dominated regime. (Imbrie et al, 1989), 3 (Hüls & Zahn, 2000), and 4 (Schmidt et al, 2004)), and sedimentary U k′ 37 (diamond 6: Herbert and Schuffert (2000)).…”

“…Such proxy-specific attributes and uncertainties can be complementary, and so we seek commonalities between coral and sedimentary paleoclimate records with the aim of better constraining LIG spatiotemporal temperature and hydrological change. Previously, fossil corals have reconstructed modern-like tropical Atlantic SSTs and δ 18 O seawater seasonality during the late (Felis et al, 2015) and early stages of the LIG, while higher than modern seasonalities were found between~123 and 126 ka (Brocas et al, 2016a(Brocas et al, , 2018a. These changes coincided concurrently with higher than modern seasonality of insolation and were indicative of Intertropical Convergence Zone (ITCZ) dynamics at that time.…”

“…Vertical error bar denote ±1 full error. (c) Bonaire coral δ 18 O seawater climatology seasonality (Brocas et al, 2018a) calculated using the Sr/Ca-and δ 18 O-SST seasonal relationship of −0.042 mmol/mol per°C and −0.196‰ per°C, respectively (Hetzinger et al, 2006). Black horizontal line is the three modern coral average with its ±1 standard deviation shaded.…”

“…Vertical error bar denote ±1 standard error. (d) Bonaire coral phase angle relationship between paired Sr/Ca and δ 18 O records (Brocas et al, 2018a). Black vertical lines are 95% confidence intervals.…”