Paper No. 0
Presentation Time: 2:30 PM
SEASONALITY, TEMPERATURE AND THE LATITUDINAL DISTRIBUTION THECIDEIDE BRACHIOPODS
JAECKS, Glenn S.1, SPERO, Howard J.
2 and CARLSON, Sandra J.
1, (1)Department of Geology, Univ of California, 1 Shields Avenue, Davis, CA 95616, (2)Department of Geology, University of California, 1 Shields Avenue, Davis, CA 95616, jaecks@geology.ucdavis.edu
Latitudinal
distributions of organisms may be limited by both mean temperature and
seasonal temperature extremes, as well as a host of other environmental
factors. Extant thecideide brachiopods are distributed throughout the tropics
and into some warm-temperate regions (up to 35 deg N). Controls on this
distribution are poorly understood. As a clade, they have broad temperature
tolerances; Pajaudina is endemic to the Canary islands where seawater
temperatures are as low as 17 degrees C, whereas Thecidellina and
Lacazella
live in waters in the Caribbean and Pacific that can reach a seasonal maximum
of 30 degrees C. Both Thecidellina and Pajaudina, as well
as Lacazella, typically inhabit waters that do not vary more than
a few degrees C annually, a phenomenon that is reflected in the oxygen
isotopic composition of their continuously accreted shell calcite. Fossil
thecideides exhibit a broader latitudinal distribution (up to paleo - 45
deg N). Have thecideide brachiopods been limited latitudinally by seasonal
extremes rather than mean temperature alone?
Thecidellina shells from Guam have a seasonal
d18O
range of 0.6 per mil, and Lacazella from the Mediterranean has a
range of 0.8 per mil. However, fossil thecideides (with no evidence of
diagenesis) exhibit greater seasonal isotopic ranges. Moorellina,
from the Jurassic of England, has a range of 2.31 per mil and Thecidiopsis
from the Cretaceous of Germany has a 1.16 per mil range. Assuming constant
local salinity and invariant d18Oseawater,
these values equate to seasonal temperature ranges of 12.5 and 6 degrees
Celsius respectively. Within the complete fossil data set, absolute d18O
values range from -0.46 to -4.42, similar to other published Mesozoic results.
Extreme negative values could result from high summer temperatures combined
with seasonal variation in salinity. Utilizing independent temperature
proxies such as Mg/Ca ratios for fossil thecideide shells may constrain
paleo d18Oseawater
and
help determine the relative roles of temperature and hydrography on shell
d18O,
and thus on thecideide distributions.