Early Permian Seawater from the delta18O Record Of Fossil Bivalves: Seasonality And A Latitudinal Gradient

• Main Author: Beard, James Andrew
• Format: Text
• Language: unknown
• Published: SURFACE at Syracuse University 2012
• Subjects: Australia; Bivalve; Eurydesma; Permian; Seasonality; Stable Isotopes; Geochemistry; Paleontology; Arctic; Arctic Ocean; Greenland
• Online Access: https://surface.syr.edu/ear_thesis/3; https://surface.syr.edu/cgi/viewcontent.cgi?article=1002&context=ear_thesis

The transition from a glaciated world to one that was ice-free makes the early Permian a time interval that in many ways mirrors the present, and hence there is great interest in constraining paleoclimate conditions over that transition. A common method for estimating ancient temperatures uses the oxygen isotope composition of marine carbonate, but this approach becomes significantly more complicated prior to the Cretaceous due to uncertainties about diagenesis and the isotopic composition of seawater, which has been hypothesized to be more depleted than during the Cenozoic. I use stable isotope compositions of sequentially microsampled accretionary calcite from fossil bivalves in SE Australia to evaluate Permian seawater isotope composition and water temperature seasonality. Co-occurring dropstones, diamicts, and glendonites constrain winter temperatures to near-freezing and hence allow calculations of water composition. Records from microsampled specimens of the bivalve Eurydesma, spanning roughly 11° of paleolatitude (North Sydney Basin, New South Wales to Hobart, Tasmania) reveal cyclic seasonal fluctuations in δ18Ocarb that vary with latitude. The δ13Ccarb values exhibit ~1 / of seasonal variation, and are in agreement with characteristically positive values published for the early Permian of ~5.5 /. The δ18Ocarb values vary seasonally by up to 3.3 /around a mean that decreases from -1.2 / to -1.75 / moving towards the pole; more enriched isotope values correspond to dark growth bands within the shells, suggesting slower growth in the winter months. Mean δ18O and seasonal amplitude both decrease with increasing paleolatitude, similar to an observed gradient in the modern high latitudes off the coast of Greenland. Decreasing seasonality is a reflection of decreasing summer temperatures with increasing latitude, while winter temperature minima are presumed to be constant because of freezing conditions. The decrease in mean δ18Ocarb with latitude reflects decreasing δ18Owater, similar to that observed over a similar latitudinal range off Greenland today. As with Greenland, the slope of the δ18O-latitude relationship is steeper than that seen in the global ocean today, indicating some contribution of isotopically negative fresh water. Whether this reflects progressive mixing with isotopically negative water from higher latitudes (e.g., the Arctic Ocean today) or similar amounts of runoff/precipitation at each location that itself is progressively more negative with latitude is as yet unclear, though significant departure from marine salinities is not observed.