North Atlantic temperature seasonality throughout the Holocene: isotopic evidence from micromilled bivalves

English Abstract:

We provide the first record of Holocene temperature seasonality for the North Atlantic, a climatically important region with significant transregional effects. Bivalves are particularly valuable proxies because growth bands of their shells archive high-resolution records of temporally discrete environmental information. High-resolution sampling enables reconstruction of variability of seasonal high and low temperatures, and therefore changes in seasonality—one of the most significant climatic variables that influences marine and terrestrial ecosystems. Thirty-five well-preserved aragonitic bivalves were extracted from a giant marine piston core in NW Iceland and sequentially sampled concordant with growth banding using a computer-controlled micromilling device. Carbonate aliquots were subsequently analyzed for δ¹⁸O_(CaCO3) values to obtain snapshots of ambient seawater temperatures at a sub-monthly resolution. Previous research on this core observed significant variations in seasonal temperature over the last two millennia (Patterson et al., 2010). Here, we extend this seasonality record back to ~10,650 cal yr BP. Our temperature record is compared to other climate records around Iceland to produce a multi-proxy reconstruction of Holocene climate variability. Our data suggest that the Early Holocene (10,650 to 7,600 cal yr BP) exhibited the most persistently warm summers, with maximum summer temperatures ~2°C higher, and with greater seasonality than during the subsequent cooler Neoglacial period (7,000 to 4,750 cal yr BP). Our results indicate that the Neoglacial period was a relatively stable cold period, with the lowest summer maximum temperatures, as well as the lowest seasonality during the Holocene. Sporadic warm periods and increased seasonality are observed in the record after 4,500 cal yr BP, when maximum summer temperatures reached between ~7 and 9.5°C. The highest reconstructed temperatures of the entire record occurred during the Roman Warm Period centered at ~2,000 cal yr BP.