Knebel O., Felis T., Asami R., Deschamps Pierre, Koelling M., Scholz D. (2024). Last deglacial environmental change in the Tropical South Pacific from Tahiti Corals. Paleoceanography and Paleoclimatology, 39 (2), p. e2022PA004585 [21 p.]. ISSN 2572-4517.
Auteurs
Knebel O., Felis T., Asami R., Deschamps Pierre, Koelling M., Scholz D.
Source
Paleoceanography and Paleoclimatology, 2024,
39 (2), p. e2022PA004585 [21 p.] ISSN 2572-4517
On glacial-interglacial time scales, changes in the Earth's orbital configuration control climate seasonality and mean conditions. Tropical coral skeletons can be sampled at a sufficient resolution to reconstruct past seasonality. Here, last deglacial Porites skeletons from Integrated Ocean Drilling Program Expedition 310 to Tahiti are investigated and, supported by a modern calibration, monthly resolved time series in geochemical proxies (Sr/Ca, delta 18O, delta 13C) are constructed. For most of the deglaciation, Sr/Ca seasonality was similar to modern (0.139 +/- 0.010 mmol mol-1; 2.8 +/- 0.2 degrees C) reflecting the small change in insolation seasonality. However, during the Younger Dryas, high values in Sr/Ca seasonality (0.171 +/- 0.017 mmol mol-1; 3.4 +/- 0.3 degrees C) suggest a reduced mixed layer depth and enhanced influence of the South Pacific Subtropical Gyre due to South Pacific Convergence Zone (SPCZ) inactivity. Furthermore, high amplitudes in Younger Dryas skeletal delta 18O (0.40 +/- 0.22 parts per thousand) and delta 13C (0.86 +/- 0.22 parts per thousand) seasonality compared to modern (delta 18O = 0.29 +/- 0.08 parts per thousand; delta 13C = 0.27 +/- 0.08 parts per thousand) point to elevated winter-summer discrepancies in rainfall and runoff. Mean coral Sr/Ca variability suggests an influence of Northern Hemisphere climate events, such as the Younger Dryas cooling (+0.134 +/- 0.012 mmol mol-1;-2.6 +/- 0.2 degrees C), or the Bolling-Allerod warming (+0.032 +/- 0.040 mmol mol-1; -0.6 +/- 0.4 degrees C). Deglacial mean coral Delta delta 18O (delta 18Oseawater contribution to skeletal delta 18O), corrected for the ice volume effect, was elevated pointing to more saline, thus dryer conditions, likely due to a northward migration of the SPCZ. Seasonal cycles in coral delta 13C were likely caused by variations in linear extension rates that were reduced during the last deglaciation (1.00 +/- 0.6 cm year-1) compared to today (1.6 +/- 0.3 cm year-1). Changes in the Earth's movement around the sun are the primary cause of changes in the annual cycle and annual mean of environmental conditions during the last glacial-interglacial periods. Distinct from most climate archives, the annual bands of coral skeletons can be sampled at a monthly resolution that allows for reconstructions of past seasonal cycles. In this study, the chemical composition of coral skeletons from the genus Porites that grew during the last deglaciation at Tahiti (similar to 15,000-9,000 years ago) was analyzed to infer seasonal variations and mean conditions in past sea surface temperature and salinity. Prior to application, the methods were successfully calibrated using modern coral skeletons from Tahiti. Results showed that sea surface temperature seasonality was similar to today during most of the deglaciation. However, for the Younger Dryas, a particular cold interval, a marked increase in sea surface temperature seasonality suggests that Tahiti was at that time located outside the South Pacific Convergence Zone, a rain band extending across the tropical Southwest Pacific. Furthermore, coral reconstructions revealed that, although distant, North Atlantic last deglacial climate events influenced mean climate conditions at Tahiti. In addition, we show that last-deglacial coral growth rates were reduced compared to today. Last deglacial changes in Tahiti coral Sr/Ca suggest an influence of Northern-Hemisphere climate events on the tropical South Pacific Variations in last deglacial Tahitian climate were modulated by meridional migrations of the South Pacific Convergence Zone Last deglacial Porites corals revealed reduced linear extension rates (1.0 +/- 0.6 cm year-1) compared to today (1.6 +/- 0.3 cm year-1)
