INTERPRETATION OF SEASONALITY FROM MARINE GASTROPOD SHELLS: LATITUDINAL CHANGES IN ISOTOPIC SIGNATURES

Latitudinal effects on marine gastropod shell growth and the potential implications for paleotemperature evaluations were studied using Oxygen stable isotopic profiles. Two Recent specimens and their fossil counterparts, a total 4 shells of comparable size (86.5 – 154 mm) of the family Fasciolariidae from low latitudes and Beringiidae from high latitudes of the northern hemisphere were selected for study. Recent Fasciolaria tulipa from 23^oN (Cay Sal Bank, Bahamas, 7 m depth) and Fasciolaria okeechobeensis from the Aftonian Pleistocene of Palm Beach County, Florida were chosen to represent the low latitude western Atlantic locality. Recent Beringius (Beringion) behringi from 47^oN (coast of Eastern Sakhalin, 200m depth), and Middle Miocene Beringius (Beringion) etolonensis from northwestern Kamchatka were selected to represent the high latitude northwestern Pacific locality. Collection sites were selected to minimize salinity effects on the isotopic composition. Samples from each specimen were taken along growth increments, starting from the protoconch area. Temperatures calculated using known d¹⁸O of ambient seawater for the recent specimens were compared with seasonal temperature and salinity data. The isotope profile of F. tulipa indicate a shell growth period of 3.5 years. Growth started in the summer, with a possible slow down during winter. Isotopic values ranging from -1.7 to 1.5‰. Calculated temperatures closely match oceanographic data. The Pleistocene F. okeechobeensis curve suggests faster growth (2.5 years, started in the summer) and similar isotopic and temperature ranges. The B.(B.) behringi curve indicate ~ 3 years of growth, started in the winter with a fast growth during the first year and winter growth cessations during 2 consequtive years. Calculated growth temperatures are slightly higher than indicated by oceanographic data. B.(B.) etolonensis shows 4 years of growth starting in late summer-autumn without clearly detected winter cessations. Differences in growth curves between recent and fossil specimens in high latitudes are attributed to the fact that B.(B.) etolonensis was living during the time of the Middle Miocene global climatic optimum, when the waters of the North Pacific were significantly warmer than today. High latitude isotopic profiles show more intra-annual variations in d¹⁸O than the low latitude profiles.