Stable Isotope Analysis of Coastal Floodplains & Ancient Greenhouse Systems
1. Using extant models to constrain predictions & 'ground-truth' approaches to isotopic reconstruction of environment and ecosystems
Similar to my research using vertebrate microfossil bonebeds to understand Cretaceous community structure and its response to environmental changes on broad scales, I am interested in performing research at finer temporal and spatial scales in order to understand environment and community structure, habitat preferences and niche-partitioning, as well as diet and trophic interactions. While my interests in these questions pertain to whole ecosystems and environments, I have a particular interest in the ecology of the dinosaurian fauna. Considerable research has been performed attempting to characterize the habitat preferences, diet, and niches of dinosaurs, and stable isotope analyses, by virtue of recording the original diets, habitats, and behaviours of these animals, offer a potential avenue for understanding those topics. However, prior to performing detailed analyses of ancient systems, we should understand the constraints that may exist on our inferences, and where possible frame our predictions based on data from similar systems today. To that end, a large component of my PhD research focused on characterizing the stable isotope ecology of vertebrate communities in both extant and ancient systems, particularly coastal floodplain forests.
(Contrasting images of Cretaceous sedimentary rocks preserving fluvial and coastal floodplain environments, and their modern near-equivalents from the Atchafalaya River Basin of Louisiana.)
First, stable isotope analyses were performed on vertebrate taxa sampled (both opportunistically and obtained through local collaborations and/or museum collections) from within the modern Atchafalaya River Basin of Louisiana, an analogue for the ecosystems and environments preserved across much of western North America during the Cretaceous (Cullen et al 2019, Cullen et al 2020). One of the major constraints limiting palaeocommunity analyses are the lack of clear modern analogues against which to test broad-scale ecological and evolutionary hypotheses, such as mechanisms of community assemblage and patterns of biogeography, within the context of deep time. Our study of the Atchafalaya Basin identified patterns of ecological resource use and aquatic-terrestrial interchange in this system, facilitated the development of improved methods of bioapatite oxygen-temperature inference, and provided an isotopic baseline for use in testing hypotheses in the fossil record.
The Atchafalaya River Basin study made use primarily of fieldwork 'opportunistically collecting' specimens, that is to say – using roadkill and other carcasses. This was surprisingly effective, though obvious some taxa are less easily locatable in this manner. As a result we also partnered with the Louisiana Department of Wildlife & Fisheries, as well as local universities, naturalists, and private citizens to obtain our full sample. Having this broad sample, with excellent observational natural history data and multiple tissue types facilitated our isotopic research.
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Nevertheless, even with well-established natural history data on ecological variables for our Atchafalaya River Basin taxa, such as trophic habits and microhabitat use, the subtleties involved in identifying these patterns in our isotopic data highlight the difficulties involved in detecting and interpreting ecological patterns in a complex C3-based system. It therefore stands to reason that for a palaeoecological system where the description of niches often requires significant inference (particularly among non-avian dinosaurs, for which there is no exact extant analogue), using isotopic data to describe fine-resolution details of niche partitioning should be done conservatively. These comparative data should assist in constraining predictions and limiting the degree to which palaeoecological-proxy data may be over-interpreted.
2. Reconstructing palaeoenvironments & testing palaeoecological hypotheses in greenhouse systems
Having established the Atchafalaya Basin as an appropriate modern comparator for a variety of Cretaceous animal assemblages opens up the potential for a variety of phenomena to be investigated such as habitat use, niche partitioning, diet, and physiology (Cullen et al 2020 Cullen et al In Review). These fine-resolution questions are rarely accessible within a palaeontological context, and the framework developed through our extant comparisons will facilitate an expansive range of future research.
From our Cretaceous stable isotope analyses, we found broad overlap exists in the isotopic distributions of large herbivorous dinosaurs (suggesting that at least in the Campanian of Alberta that these animals were likely not partitioning their niches exclusively through differences in habitat use), and that much like in extant coastal floodplain there was considerable aquatic-terrestrial resource interchange in these Cretaceous communities. Additionally, through my modern isotopic studies in Louisiana, I developed a novel and surprisingly accurate approach to estimating mean environmental temperatures using bioapatite δ18O, reducing estimation variability by incorporating data from a physiologically and ecologically broad sample of vertebrates (Cullen et al 2019). I then applied this method to coastal plain deposits in the Campanian of Alberta, reconstructing new palaeoenvironmental temperature conditions using oxygen isotope data, while facilitating the estimation of isotopic niches and habitat use in this system, alongside comparisons to modern isotopic niches (Cullen et al 2020).
From our Cretaceous stable isotope analyses, we found broad overlap exists in the isotopic distributions of large herbivorous dinosaurs (suggesting that at least in the Campanian of Alberta that these animals were likely not partitioning their niches exclusively through differences in habitat use), and that much like in extant coastal floodplain there was considerable aquatic-terrestrial resource interchange in these Cretaceous communities. Additionally, through my modern isotopic studies in Louisiana, I developed a novel and surprisingly accurate approach to estimating mean environmental temperatures using bioapatite δ18O, reducing estimation variability by incorporating data from a physiologically and ecologically broad sample of vertebrates (Cullen et al 2019). I then applied this method to coastal plain deposits in the Campanian of Alberta, reconstructing new palaeoenvironmental temperature conditions using oxygen isotope data, while facilitating the estimation of isotopic niches and habitat use in this system, alongside comparisons to modern isotopic niches (Cullen et al 2020).
(O-isotope temperature estimates from bioapatite Cretaceous system [compared to other estimates] and modern coastal floodplain systems ['ground-truthing' of accuracy], Cullen et al 2020)
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(comparisons of Cretaceous & modern near-analog coastal floodplain vertebrate communities in C-O stable isotope space, Cullen et al 2020)
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In addition to these ecological and habitat/environment studies, the combination of the Louisiana and Cretaceous data allow additional questions to be answered. A long-documented anomalous positive enrichment of δ13C in fossil dinosaur tissues was suggested to indicate a unique dietary physiology, which has significant ramifications for our understanding of their ecology and evolution. However, this assertion had never been rigorously tested. I performed intensive stable isotopic analyses of dinosaurs and physiologically distinct non-dinosaurs (fish, mammals, reptiles) from the same Cretaceous palaeoenvironment to test this hypothesis, while also comparing to a similar range of animals from Louisiana as a control. I found the positive δ13C anomaly present in not only dinosaurs, but all sampled Cretaceous taxa and in no living relatives, even after controlling for diagenetic alteration (Cullen et al In Review), concluding that this anomaly actually represents a shift in the carbon isotopic baseline in this greenhouse environment relative to modern systems. This result underscores the need for more research in reconstructing greenhouse palaeoenvironments, particularly in what ways they are non-analogous with modern environments, if they are going to be meaningfully compared to modern conditions. A note though is that these results are under peer-review for publication. Additional details will be posted soon as this paper and some related work are accepted for publication.
Currently, I am using a suite of non-traditional stable isotope and geochemical analyses to study the environments and ecosystems of the Late Cretaceous in even greater detail, in order to test hypotheses of niche-partitioning, habitat use, and trophic ecology to a degree of resolution not previously known
Currently, I am using a suite of non-traditional stable isotope and geochemical analyses to study the environments and ecosystems of the Late Cretaceous in even greater detail, in order to test hypotheses of niche-partitioning, habitat use, and trophic ecology to a degree of resolution not previously known