Stable Carbon and Nitrogen Isotopes of Shell Bound organic Matter, Implications for Modern and Ancient Food Web Dynamics
Date of Award
Doctor of Philosophy (PhD)
ammonite, bivalve, carbon, nitrogen, stable isotope, trophic level
Earth Sciences | Physical Sciences and Mathematics
Stable isotopes are an important tool for solving biogeochemical problems and have been increasingly applied in ecosystem studies because of their utility as source-to-sink tracers and for ecological information. Elements like C and N have more than one stable isotope, and their isotopic compositions (δ15N and δ13C) vary in a foreseeable way and are specifically useful in studying global elemental cycles and energy flow through ecosystems. δ15N and δ13C of organic matter (OM) are diagnostic of an organism's diet and have been used as a proxy for trophic level reconstructions. However, because of several analytical constraints, the use of stable carbon and nitrogen isotopes for trophic studies is still in an early stage, and there are several novel prospective avenues where they can be applied. In this dissertation, I aim to test the applicability of this approach for high resolution paleoecological studies using OM preserved in the shells of marine invertebrates. δ15N values of an organism's tissue is dependent on the δ15N of its food source, and typically exhibit an enrichment of around 3.4‰. Based on this discrimination factor, a stepwise increment is expected as we move higher in the food chain. However, this estimate is complicated by two factors: 1) variable isotopic composition at the base of the food web that changes on spatial and temporal scales and 2) trophic omnivory, where an organism feeds at multiple trophic levels. As a result, the δ15N signature of an organism's tissue can incorporate these temporal changes in its isotopic composition and complicates the process of extracting trophic information from an individual. Bivalves form their shell via accretion which makes them an important environmental archive, especially for tracking baseline isotopic changes temporally. The bivalve shell also contains an organic matrix, that serves as a nucleation site for calcium carbonate crystals and is mediated by the mantle tissue. Thus, the shell-associated organic matter should also display δ15N values that are representative of the soft-tissue of the bivalve. Unlike soft-tissue, the skeletal hard part of an organisms is easily fossilized and allows for isotopic investigation of long-extinct taxa. Since primary production vary at a seasonal time scale, different environments go through seasonal changes in their isotopic baseline and subsequently affects the isotopic signature of consumers. Suspension feeding bivalves, which feed on sedimenting particles in the water column provide an ideal system to investigate these changes. In Chapter 1, I use high-resolution sclerochronological data from recent and live bivalve Spisula solidissima, to investigate variation in the δ15N values over ontogeny. Our results demonstrate that the δ15N values of a bivalve shell incorporates seasonal variation and exhibits a range of values that are similar to the trophic discrimination factor of 3.4‰. We also demonstrate that one way to circumvent the problem is by analyzing multiple specimens or fragments, to arrive at a mean δ15N value at the population level. This study is one of the first sclerochronological investigations of δ15N for the entire ontogeny of a bivalve and was possible by the implementation of a novel nano-EA technique that requires significantly lower amount (~1000 times) of carbonate samples. This chapter also supports the applicability of this method using fossil molluscs where extractable organic matter could be in concentrations too low for conventional nitrogen isotope analysis. In Chapter 2, I use the nano-EA technique to extract trophic information about a long-extinct enigmatic heteromorph ammonite, Baculites. Contrary to the usual understanding that ammonites were nektonic carnivores, δ15N and δ13C analysis of shell associate OM from Baculites and co-occurring ammonite Hoploscaphites reveal that they were planktivores and actively fed on small organisms like decapod crustaceans, copepods and ammonite larvae. Although several studies in the past attempted to understand the feeding habit of Baculites by analyzing the morphology of its feeding apparatus and imaging of its buccal mass, this study is the first one to use δ15N to investigate its feeding behavior. The study is also a test of concept for using the nano-EA method for extracting trophic information from long-extinct fossils. In Chapter 3 then, I use stable isotope (δ15N and δ13C) data from shell associated organic matter of multiple taxa to reconstruct trophic structure of a well-preserved shallow-marine fossil deposit from the Late Cretaceous of Antarctica. I use the δ15N value to estimate the trophic position of individual taxa and δ15C to identify the source of carbon at the base of the food web. The range of δ15N values exhibited by the specimens suggest that three distinct trophic levels are present and δ15C values point towards a single source of carbon at the base of the food web. Taxon-specific δ15N values did not always concur with the inferred trophic assignment based on conventional trophic approach, suggesting that complex trophic interaction like omnivory, intraguild predation, and cannibalism can complicate the traditional approach to estimate trophic position. This dissertation corroborates the use of nano-EA method to study isotopic signatures from shell-associated organic matter not only from modern organisms but also from well-preserved fossils. Our ability to acquire meaningful δ15N and δ13C data by utilizing only a fraction of what is required by conventional δ15N and δ13C analysis opens new possibilities for answering important paleoecological questions, especially when dealing with museum specimens that are limited for destructive analysis. This study also provides a basis for using δ15N and δ13C to infer feeding habit and trophic position of long-extinct fossils. This approach to estimate trophic structure using isotopes, combined with major community properties like biovolume and abundance can shed light into the community structure of fossil ecosystems and can illustrate a how communities respond to major purturbations in the global biogeochemical cycle.
Das, Shibajyoti, "Stable Carbon and Nitrogen Isotopes of Shell Bound organic Matter, Implications for Modern and Ancient Food Web Dynamics" (2022). Dissertations - ALL. 1493.