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Variability of oxygen isotopes in environmental water is recorded in tooth enamel, providing a record of seasonal change, dietary variability, and mobility. Physiology dampens this variability, however, as oxygen passes from environmental sources into blood and forming teeth. We showcase two methods of high resolution, 2-dimensional enamel sampling, and conduct modeling, to report why and how environmental oxygen isotope variability is reduced in animal bodies and teeth. First, using two modern experimental sheep, we introduce a sampling method, die-saw dicing, that provides high-resolution physical samples (n = 109 and 111 sample locations per tooth) for use in conventional stable isotope and molecular measurement protocols. Second, we use an ion microprobe to sample innermost enamel in an experimental sheep (n = 156 measurements), and in a Pleistocene orangutan (n = 176 measurements). Synchrotron and conventional μCT scans reveal innermost enamel thicknesses averaging 18 and 21 μm in width. Experimental data in sheep show that compared to drinking water, oxygen isotope variability in blood is reduced to 70–90 %; inner and innermost enamel retain between 36 and 48 % of likely drinking water stable isotope range, but this recovery declines to 28–34 % in outer enamel. 2D isotope sampling suggests that declines in isotopic variability, and shifted isotopic oscillations throughout enamel, result from the angle of secretory hydroxyapatite deposition and its overprinting by maturation. This overprinting occurs at all locations including innermost enamel, and is greatest in outer enamel. These findings confirm that all regions of enamel undergo maturation to varying degrees and confirm that inner and innermost enamel preserve more environmental variability than other regions. We further show how the resolution of isotope sampling — not only the spatial resolution within teeth, but also the temporal resolution of water in the environment — impacts our estimate of how much variation teeth recover from the environment. We suggest inverse methods, or multiplication by standard factors determined by ecology, taxon, and sampling strategy, to reconstruct the full scale of seasonal environmental variability. We advocate for combined inverse modeling and high-resolution sampling informed by the spatiotemporal pattern of enamel formation, and at the inner or innermost enamel when possible, to recover seasonal records from teeth. 

Understanding the transport mechanisms of terrestrial biomarkers to marine sediments is critical for interpreting past environmental and climate changes from these valuable archives. Here, we produce new estimates of two classes of terrestrial plant biomarkers, n-alkane waxes and pentacyclic triterpene methyl ethers (PTMEs), from a transect of marine core top sediments that span the full length of the West African margin. We determine the chain length distributions, mass accumulation rates, carbon isotope signatures (δ13C) of n-alkanes and the mass accumulation rates of PTMEs and assess the extent to which these proxy characteristics reflect vegetation and climate patterns within source areas on adjacent land. We achieve this via comparisons with a variety of satellite-based vegetation and climate data sets and with atmospheric back trajectory and river basin estimates. The mass accumulation rate of grass-produced PTMEs to core top marine sediments shows good spatial agreement with the presence of C4 grasses on land and appears to have shorter transport distances than n-alkanes. The mass accumulation rate of n-alkanes roughly corresponds to changes in the landscape net primary productivity. The δ13C signature of n-alkanes records changes in landscape C3 versus C4 vegetation balance while longer chain length n-alkane distributions indicate drier conditions and grassier vegetation. Apparent discrepancies between the zonal distribution of biomarkers in the marine sediments versus the observed vegetation patterns can mostly be explained by the influence of long-range atmospheric transport, with modest contributions from river inputs. 

Nearly 100 million people live in and depend on the Sahel for agriculture and natural resources. The region is sensitive to natural climate and environment variations caused by the seasonal movement of the tropical rainbelt. In the paleoclimate record, insolation plays a clear role on West African Monsoon strength, but responses to other forcings like temperature, greenhouse gases, ice volume, and land surface cover are unclear due to the lack of highly resolved, terrestrial records that span major global and regional shifts through time. Here we present leaf wax precipitation and vegetation records from several targeted study windows throughout the last 25 million years, derived from long-chain n-alkane hydrogen (δDwax) and carbon (δ13Cwax) isotopes, respectively, in a sediment core from ODP Site 959 in the Gulf of Guinea, where westerly winds and major river systems transport Western Sahel-sourced material. Analyses of trend and variability document a range of rainfall and vegetation responses to orbital forcings in different boundary conditions in the Oligocene, Miocene, Pliocene, and Pleistocene. We find that both the climate and environment was more variable in times of higher CO2 and global temperatures, suggesting an increase in ecosystem instability moving forward into the future. Because of the high resolution and temporal coverage of these new biomarker isotope records, we can examine relationships between precipitation and vegetation fluctuations, even prior to C4-expansion when there was a strong correlation despite minimal variation in δ13Cwax in a C3 world. Further, we find a wetting trend throughout the record, demonstrating that vegetation on long timescales was decoupled from hydroclimate and that the terrestrial ecosystem may face aridification, contradicting some model projections. 

Wildfires are essential to terrestrial ecosystems, playing a crucial role in nutrient and carbon cycles, particularly in highly seasonal environments like the Western Sahel. Their occurrence is linked to complex feedback mechanisms between climate, landscape structure, vegetation and the carbon cycle. It is therefore central to understand wildfire dynamics in the context of paleoclimatic and environmental change. Here we present a record of 3 to 7 ringed polyaromatic hydrocarbons (PAHs), from five targeted study windows throughout the last 25 million years from ODP Site 959 in the Gulf of Guinea. The time windows target the effects of orbital forcings of the West African Monsoon on wildfire and vegetation responses in different boundary conditions in the Oligocene, Miocene, Pliocene, and Pleistocene, including shifts in global temperatures, greenhouse gas concentrations, and regional land surface. Orbitally resolved PAH biomarkers can provide insight into fire activity and be coupled with changing precipitation patterns and biomes. We discuss PAH sources and how wildfire frequency is linked to the observed drying trend, climate variability, and vegetation expansion throughout the Cenozoic in the Western Sahel. These findings are central for understanding future wildfire dynamics in the vulnerable Western Sahel region in the light of global warming. 

Reconstructions of eastern African vegetation and climate are critical for understanding primate and large mammal evolution in the Neogene. Insight into past ecological conditions can be gleaned from lipid biomarkers preserved in sedimentary archives, providing evidence for the role of habitats (e.g. open vs. closed vegetation) on evolutionary trait selection. A common paleoecological proxy is the 𝛿¹³C of n-alkanes, which integrates the distinct isotopic signatures of C3 and C4 vegetation. In typical modern tropical ecosystems, “woody” vegetation uses C3 photosynthesis while “grassy” vegetation uses C4 photosynthesis. Under these conditions, mixing models can then estimate the fraction of woody cover of a landscape. While the use of photosynthetic pathways to infer plant functional type (PFT) is powerful, this paradigm does not hold prior to the rise of C4 grasses at 10 Ma, leaving a gap in understanding of ecosystem structure in the early-mid Miocene. To address this issue, we investigate whether n-alkane chain length distributions (rather than 𝛿¹³C) hold information about plant functional type independent of photosynthetic pathway. Here, we present n-alkane chain length data from over 800 modern plant samples, representing a variety of different photosynthetic pathways, growth forms, habitats, and locations. This dataset comprises a significant literature review component, as well as over 400 new distributions generated in this study. We build upon our previous work using PCA and turn to non-linear methods – including both supervised neural network classifiers and unsupervised dimensionality reduction – to determine the potential of n-alkane distributions for PFT identification. Successful differentiation between woody and grassy PFTs using modern plant n-alkane chain lengths will provide a foundation for applying this tool to the geologic record. Our method will compliment well-established isotopic measurement practices while offering the novel ability to reconstruct vegetation structure in pure C3 ecosystems. This represents a particularly powerful tool for understanding ecological history prior to the rise of C4 grasses.