Impact of environmental conditions on geochemical proxies in tropical marine calcifiers

Abstract

Skeletal structures of marine calcifying organisms have become an important archive of ancient environmental conditions. By the means of ontogenetic geochemical signatures (so-called a proxiesa ), in particular accretionary calcareous skeletal structures such as corals, bivalve shells or fish otoliths provide sub-seasonally resolved records of modern and ancient climate variability. However, complex physiological and environmental processes can complicate these geochemical signatures and a thorough understanding of these mechanisms is a prerequisite for reliable paleoenvironmental reconstructions. By focusing on proxy incorporation pattern in modern and archeological specimens this thesis aims to extend the applicability of geochemical proxies of calcareous skeletal structures for reliable paleoclimate reconstructions. First, the influence of water temperature on oxygen isotopes, carbon isotopes as well as Mg/Ca ratios in modern echinoid spines (Eucidaris galapagensis) sampled across the natural water temperature gradient of the Galapagos archipelago was studied. Thereby, it was shown that in addition to water temperature, additional physiological or environmental variables affect the incorporation of in particular carbon isotopes and Mg/Ca ratios. Nevertheless, the significant correlation of oxygen isotopes measured in E. galapagensis spines and measured water temperatures throughout the Galapagos archipelago indicates their potential for paleoenvironmental reconstructions. Secondly, this thesis provides new insights into the applicability of mathematical approaches for the conversion of ontogenetically distorted isotope records from fish otoliths and bivalve shells into sub-seasonally resolved time series. Using new simple but traceable linear interpolation approaches as well as more complex growth models from fisheries biology, ontogenetically distorted otolith and bivalve oxygen isotope records were successfully converted into time series and correlated against measured water temperatures. The resulting significant correlations indicate that ontogenetic oxygen isotope records of both skeletal structures represent reliable archives of (paleo-) environmental conditions. Within a third study, the effect of pre-historic cooking methods on proxy signatures of aragonitic skeletal structures from archeological deposits has been tested using an experimental approach. The results show that all prehistoric cooking methods cause an alteration of the shell chemistry in aragonitic bivalves with regard to the most commonly used geochemical proxy systems even without a conversion of primary aragonite into secondary calcite. However, clumped isotope analysis showed that this proxy system represents a so-far unique tool to detect and reconstruct certain pre-historic cooking methods and therefore help to avoid erroneous paleoclimate reconstructions from archeological deposits. Finally, ontogenetic oxygen isotope records of archeological fish otoliths and bivalve shells were used to reconstruct mid- to late Holocene environmental conditions along the Banc d Arguin, Mauritania, NW Africa. By showing unreasonably low oxygen isotopic compositions, these skeletal structures from a large paleo-estuary indicate consistently the persistence of isotopically lighter monsoon-related freshwater discharge into the coastal zones during the mid-Holocene. Given their deposition a few hundred years after the supposed abrupt aridification of NW Africa, the data questions the hypothesis of an abrupt termination of the African Humid Period. Aside from showing the potential of geochemical proxies from echinoid spines for paleoenvironmental reconstructions, this thesis underlines the unique potential of ontogenetic oxygen isotope records of modern and archeological otoliths and bivalve shells for sub-seasonally resolved (paleo-) climate reconstructions if the samples are adequately preserved.