Novel molecular isotope proxies in bivalves for reconstructing spatial and temporal biogeochemical cycling in marine ecosystems
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Novel molecular isotope proxies in bivalves for reconstructing spatial and temporal biogeochemical cycling in marine ecosystems


Novel molecular isotope proxies measured in bivalve tissues represent a wide range of approaches for tracing ecological and climatological change through time and space. In comparison to other oceanographic data used to reconstruct modern and past biogeochemical cycles (e.g., marine sediment cores), chemical signals preserved in sessile, filter-feeding mollusks record climate and ecological signals directly in the context of a local environment. As global planetary warming accelerates climate change, marine ecosystems are exhibiting vastly different responses in terms of nutrient availability, phytoplankton community composition, and fundamental baseline biogeochemical cycles. Responses are particularly dynamic in highly variable coastal regions, underscoring the need for creating detailed, local historical and geologic records to understand how specific ecosystems and regions have responded to past climatic regime shifts. In my thesis, I have developed a set of novel approaches that couple bulk and compound-specific isotopes of amino acids (CSI-AA) in multiple bivalve tissues (shell and soft tissue) to reconstruct the integrated biogeochemical histories of environment in which bivalves grow. These new integrative isotope techniques are particularly valuable for environments with extreme variability (e.g., nearshore margins), those that are very difficult to sample (e.g., the deep-sea), or as new paleo reconstruction approaches which can be used with any reasonably preserved shell sample from either sediment cores or archaeological sites. Key findings include: the development of a novel suite of geochemical proxies for tracing chemosynthetic production in spatially heterogenous, deep-sea methane cold seeps, including for the first time quantifying the amount of nitrogen obtained through heterotrophic filter-feeding vs. chemoautotrophy in a chemosymbiotic bivalve (Chapter 1). These new geochemical proxies can now be applied to any remnant bivalve shell to reconstruct the biogeochemical histories of often times transient methane seep systems. Next, I tested and calibrated CSI-AA proxy approaches in shell matrix protein (Chapter 2) to develop preserved bivalve shell as a bioarchive across multiple environments. I compared isotope patterns in three different bivalve species from two coastal ecosystems (littoral and estuary) and show that the well-established ecological isotope proxies (niche width, baseline δ13C and δ15N, trophic level, and resource contribution) calibrated in bivalve soft tissue are directly transferred and preserved to shell protein matrix. However, in this chapter I also show that past CSI-AA trophic level calculations are fundamentally inaccurate and propose a new mollusk-specific trophic level equation required for both soft tissue and shell, linked to consistently compressed trophic discrimination factors and shell isotope routing. Finally, to determine the fidelity of isotope signals in ancient bivalve shell, I tested the preservation of bulk and CSI-AA isotope values and patterns in a suite of archaeological shells spanning a wide range of time periods between the middle to late Holocene and depositional preservation environments (Chapter 3). I show that even in subfossil shells whose bulk isotopes are strongly diagenetically altered, CSI-AA proxies are intact across almost 6 kyr of preservation. This work quantitively demonstrates CSI-AA preservation in the insoluble shell matrix protein of bivalves and sets the stage for using bivalve shell as bioarchives for CSI-AA parameters, to reconstruct paleoecological histories of nearshore systems with far more detail and precision that has ever been possible.

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