Seasonal Variations Preserved in an Extinct Neogene Scallop, Chesapecten, from Florida to Delaware, USA and its Implications for Paleobiogeography
Abstract
High-resolution records of climate variability on deep-time scales are needed to advance our understanding of the impact of a warming climate on seasonality and ecological change along a latitudinal gradient. The Middle Pliocene Warm Interval (MPWI: 3.2-2.8 Ma) provides insight into a globally warmer world, in which, relative to today, continental and oceanic configurations and atmospheric CO2 levels were similar; sea and continental ice were reduced; and interiors of continents were arid. Accretionary hard parts of marine organisms serve as physical (growth lines and increments) and chemical (87Sr/86Sr, δ18 O and δ13C) archives of life history, ecology, and environmental conditions during the life of the animal. Our goal was to examine variations in seasonality across latitudinal (~27° to37°N) and biogeographic (tropical to cold-temperate) gradients of the Middle Atlantic Coastal Plain (MACP) during two intervals of warming: the Middle Miocene Climate Optimum (MMCO: 17-15 Ma) and the Middle Pliocene Warm Interval (MPWI: 3.2-2.8 Ma). We analyzed variations in annual shell growth and isotope ratios (87Sr/86Sr, δ18O and δ13C) of 12 Chesapecten shells from the extremes of their biogeographic range (tropical to warm- temperate) (MMCO: Florida and Delaware; and MPWI: Florida and Virginia). Today, Florida is warm-temperate, and Delaware and Virginia are cold-temperate. Chesapecten are an extinct genus of scallop commonly preserved in MAPC deposits. They inhabited subtidal marine environments during the Miocene and Pliocene. We used 87Sr/86Sr ratios to tightly refine the timing and modeled age of the MMCO and MPWI. Modeled ages across Chesapecten's biogeographic extremes include: Florida (MMCO) 15.5 to 14.1±0.6 Ma; Delaware (MMCO) 18.0 to 17.5±0.4 Ma; Florida (MPWI) 3.75 to 2.05±0.9 Ma; and Virginia (MPWI) 2.45 to 1.65±0.4 Ma. We estimated seasonal temperature from the δ18O time series assuming interglacial δ18OSEAWATER values of -0.05‰ and -0.35‰ for the MMCO (Florida and Delaware, respectively) and 0.96‰ and -0.50‰ for the MPWI (Florida and Virginia, respectively), accounting for latitudinal effects. Temperatures during the MMCO from Florida shells ranged from 19.7 to 31.4°C, and from Delaware shells ranged from 13.3 to 26.1°C. These data reflect diminished seasonality from Florida to Delaware at that time. Prominant annual growth lines were not apparent in these shells, a common characteristic in tropical shells. Like the MMCO temperature estimates, temperature recorded in MPWI shells from Florida lacked seasonal variability, as expected from low latitudes, with temperatures ranging from 15.6 to 21.3°C. Furthermore, like the MMCO shells, prominent annual growth lines were not evident. MPWI shells from Virginia displayed seasonal temperatures from 2.7 to 20.1°C indicating warm- temperate conditions. We predicted annual growth line formation during summer months based on studies that show modern warm-temperate bivalves inhabiting mid to high latitudes form annual growth lines during summer months. However, isotope sclerochronology revealed that these scallops produced winter growth lines. These findings have important implications for paleoceanographic and atmospheric circulation during the MPWI.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- Bibcode:
- 2007AGUFMPP51E..05G
- Keywords:
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- 1605 Abrupt/rapid climate change (4901;
- 8408);
- 1616 Climate variability (1635;
- 3305;
- 3309;
- 4215;
- 4513)