Fossil records of the cold‐water scallopZygochlamys delicatula(Mollusca: Bivalvia) off northernmost New Zealand: How cold was the Last Glacial maximum?

Abstract

Three new localities for fossil Zygochlamys delicatula (Hutton) are reported: 20 km north of the Three Kings Islands, at 812 m depth in surface sediment (late Last Glacial age, 14 319 ± 86 radiocarbon years BP); and off Ninety Mile Beach in c. 170 m, and “a little north of Auckland” in c. 350 m, northwestern North Island, both in brown mixed phosphate‐carbonate concretions (pre‐Last Glacial; possibly as old as early Nukumaruan?). The most northern locality is >600 km north of the previously northernmost fossil record, in early Nukumaruan rocks of central Hawke's Bay, and c. 1000 km north of the northernmost abundant living specimens, in Pegasus Canyon, north of Banks Peninsula. Planktonic larvae of Z. delicatula appear to be limited to a summer maximum sea surface temperature no warmer than c. 14–15°C. A drop in summer maximum sea surface temperature of at least 6°C during the Last Glacial maximum therefore appears to be needed to allow Z. delicatula to occur northwest of the Three Kings Islands. Enhanced upwelling nearshore off western Northland and off northernmost New Zealand during the Last Glacial maximum, perhaps caused by enhanced wind flow, apparently allowed rare spatfalls of Z. delicatula to metamorphose and survive to adulthood off the Three Kings Islands. Z. delicatula might well have been quite common off western Northland, but if so is now largely buried beneath the seafloor.