16-4 Using Elemental and Isotopic Geochemical Proxies to Investigate the Ecology of Fish From Lower Jurassic Rift Lakes in Massachusetts and Connecticut

Session: From Thin Section to Outcrop: Exploration of Undergraduate Research (Posters)

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Abstract:

The Deerfield and Hartford Basins of Massachusetts and Connecticut bear highly-cyclical lacustrine deposits from the Lower Jurassic, with lake highstands represented by distinct beds of organic-rich mudstone and minor carbonates. These beds have produced rich and well-preserved fossil fish assemblages, containing the holostean Semionotus, the palaeonisciform Redfieldius, the ptycholepiform Ptycholepis, and the coelacanth Diplurus. Changes in the bulk organic carbon isotope ratio (δ¹³C) of the sediment within individual lake cycles have previously been interpreted as reflecting δ¹³C gradients within the lake, controlled by water depth and meromixis. Stratigraphic changes in the abundance of each fish genus have also been recorded within lake cycles and correlated with sediment δ¹³C, suggesting paleoenvironmental controls on the distribution of different taxa. However, the isotopic composition of the fish fossils themselves has yet to be investigated. In this study, we employ several geochemical proxies to assess niche partitioning in the genera Semionotus and Redfieldius from the Shuttle Meadow Formation of the Hartford Basin, Connecticut. We measure the δ¹³C value of organic matter from preserved soft tissues as well as δ¹³C and δ¹⁸O from bioapatite structural carbonate in bony scales. Additionally, we measure mercury concentration ([Hg]) in fish organic matter to test for bioaccumulation and assess the utility of this proxy to infer relative trophic level. In modern lacustrine ecosystems, planktonic/pelagic primary producers tend to be more ¹³C-depleted than benthic/littoral producers, and this difference is also reflected in higher-level consumers such as fish. Thus, we hypothesize that Redfieldius exhibits a lower average δ¹³C value than Semionotus due to its morphological similarity to planktonic ram-feeders and greater abundance in the deepest-water portions of the lake cycle. We also hypothesize that Redfieldius exhibits lower and less-variable [Hg] values due to feeding at a lower trophic level than Semionotus, which has been interpreted as a generalist suction feeder. Fish fossils from the Mesozoic rift basins of eastern North America bear a largely-unexplored geochemical record that can provide insights into paleoecology around the Triassic/Jurassic boundary.

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