Changes in biological production in the mixed water region (MWR) of the northwestern North Pacific during the last 27 kyr

Piston core PC6 (40°N, 143°E) provides the first record of biological production and alkenone sea surface temperature (SST) in the mixed water region (MWR) off Sanriku, Japan, in the northwestern North Pacific over the last 27 kyr. Biogenic opal content was low until the early Holocene, and rapidly increased at 6 calendar kiloyears before present (cal kyr BP). Low biological production in the MWR resulted from the reduced duration of the phytoplankton bloom during the glacial period (27–18 cal kyr BP), and from nutrient depletion, especially silica, during the deglaciation (15–9.5 cal kyr BP). In spite of the decreased amount of nutrients in the Oyashio water, biological production increased in the late Holocene (6 cal kyr BP–present). The data revealed that nutrients supplied from deeper levels by eddies played an important role in biological production in the MWR, contrary to the conventional concept that the Oyashio current has controlled productivity in this region. Alkenone-SST during the glacial period was on average 13.6 °C, which is close to the modern SST (14.6 °C) in June, but this timing is not consistent with the CLIMAP results (13.4–14.3 °C in August). This result indicates that alkenone-SST recorded not late spring but summer temperatures, owing to a bloom shift caused by surface water cooling during the last glacial period. Alkenone-SST was even lower during the deglaciation than during the LGM and reached a minimum (9.4 °C) at 14.0 cal kyr BP. δ15N was very high during the deglaciation, implying that nutrients in the euphotic zone were depleted around the core site. Nutrient stress probably had a physiological impact on coccolithophorids and resulted in the apparent minimum alkenone-SST. These findings indicate that the reconstruction of SST by alkenones can sometimes be very complex and possibly misleading.