Permalink : http://hdl.handle.net/2115/38444

KEYWORDS : Bryozoa;Ecophenotypic response;Growth;MART;Paleoseasonality;SL-MART;Temperature

Zooids in bryozoan colonies show an ecophenotypic response in which zooid size varies inversely with temperature at the time of zooid growth. A technique called MART analysis allows estimation of the mean annual range of temperature (MART) in Recent or paleo- marine environments by measuring the mean coefficient of variation (CV) of zooid size in species of cheilostome bryozoans that lived in those environments. We conducted standard MART analyses of Recent specimens from nearshore shelf regions in Akkeshi Bay (Japan) and the western Aleutian Islands (Alaska) as controls on our application of the method, and of fossil specimens from a Pleistocene shallow benthic environment [Nakasato Congomerate Member, Utasai Section, lower Setana Formation (1.2~1.0 Ma), Kuromatsunai, Hokkaido, Japan] to examine paleoseasonality. In some cases, not enough material was available from fossil strata to conduct standard MART analyses. For these cases, we developed a modified technique that we call specimen-limited MART analysis, or SL-MART. We applied this technique to specimens from two stratigraphically sequential sediment samples from the Soebetsu Sandstone Member in the Soebetsu Section of the upper Setana Formation (1.0-0.6 Ma). Regardless of technique or age of the specimens, replicate samples in our analyses provided remarkably similar estimates of seasonality. The utility of the controls was questionable due to uncertainty about present-day temperature ranges in the localities sampled. A decrease in average MART value from the lower to upper Setana Formation (11.8℃ and 8.6℃, respectively) indicates more pronounced seasonality in the lower than the upper Setana, perhaps suggesting a shift to colder conditions in the latter. However, the younger samples could also have lived at a greater depth, which would also reduce the MART. In addition, previous studies of marine mollusks indicate variability in paleoenvironment within both the upper and lower Setana Formation, perhaps related to the warm Tsushima current that flowed into the Sea of Japan during most interglacial periods from 1.71-0.8 Ma. Thus more than a few scattered MART estimates are necessary for reconstruction of paleoseasonality through the depositional interval of the Setana Formation.