COASTAL CALIFORNIA CLIMATE RESPONSE TO THE 8.2 KYR EVENT: A HIGH-RESOLUTION, MULTI-PROXY SPELEOTHEM STUDY

The 8.2 kyr event is an abrupt cooling during the Holocene thought to be caused by a decline in North Atlantic thermohaline circulation due to a large pulse of fresh water. This disturbance is clearly noted in Greenland ice core records and observed in sediments in the subtropics and tropics. Due to the brevity of the event, it is difficult to capture in many archives. Speleothems have the potential to provide high-resolution, well-dated records of early Holocene climate and thus can capture the terrestrial response to this event. Speleothem oxygen isotope records from China, Oman, and Costa Rica record decreased monsoon intensity during this time, while Brazilian speleothems record increased monsoon intensity, suggesting a southward shift in the Intertropical Convergence Zone. Paleoclimate records along the California coast indicate decreased sea surface temperatures; however, little is known about the effects of the event on terrestrial climate in the region. Understanding the response of California climate to the 8.2 kyr event could provide insight into the regional response to freshening of the North Atlantic due to melting of the Greenland ice sheet with global warming.

Here we present annually resolved δ¹⁸O and δ¹³C, and sub-annually resolved trace element records from a speleothem collected from a cave in the Santa Cruz Mountains. Stalagmite SC-1 grew from 8.7 ± 0.03 to 7.1 ± 0.03 kyr, across the interval in which the 8.2 kyr event occurred. SC-1 shows evidence of rapid continuous growth (~100 µm/yr) and confocal laser fluorescent microscopy reveals quasi-annual banding throughout the sample. At 8.23 ± 0.04 kyr there is an abrupt shift to lower δ¹³C and δ¹⁸O and Mg concentrations. δ¹³C, δ¹⁸O, and Mg are positively correlated, with shifts in δ¹³C leading Mg and δ¹⁸O by 6 and 20 years, respectively. Concentrations of Sr and Ba also decrease at this time, while, U, P, and Y increase. These proxies suggest decreased water rock interactions and prior calcite precipitation and increased soil respiration and input of soil-associated elements, indicating a generally wetter climate on the California coast. The shift to more negative δ¹⁸O indicates more North Pacific sourced storms reached the cave at this time. The results of this study imply that the California coast experienced a cooler, wetter climate as a result of the event.