Predicting the future of northern North Atlantic shallow water ecosystems from fossil bio-archives

Abstract

A comprehensive understanding of past climates and environmental conditions is essential for our ability to successfully predict the likely impacts of future climate change and to effectively employ adaptation and mitigation strategies. The continued improvement and verification of numerical global circulation models (GCMs) is our best tool for predicting future climate scenarios. However, longer time-scale reconstructions particularly rely on archives (e.g., ice cores, sediment cores) that contain preserved records of past conditions (proxies) in their biogenic hard parts (e.g., shells, bones, teeth) or non-biogenic deposits/accumulations (e.g., lake varves, layers in stalagmites or ice cores). The marine bivalve Arctica islandica is one such versatile and commonly used biogenic archive. Its key advantages are its longevity (up to 500 years old), its wide distribution throughout the North Atlantic and its abundance in the fossil record (back to 20 Ma). Variability in shell growth and the biogeochemistry of its shell carbonate (aragonite) enable reconstructions of, for example, large-scale ocean-atmosphere phenomena such as the North Atlantic Oscillation and sub-annual (seasonal) absolute water temperatures (based on stable oxygen isotopes). This well understood and wellcalibrated archive has most commonly been used for environmental reconstructions in the last 2,000 years but its full potential for climate reconstructions further back in time has not yet been fully exploited. Therefore, this thesis aims to thoroughly assess the potential of sub-fossil (up to 10,000 years old) and fossil A. islandica shell specimens as a palaeo-archive that records the environmental conditions of the North Atlantic during past warm phases. The use of any proxy data from fossil specimens presents challenges that can significantly impact upon palaeo-environmental interpretations. I firstly present a new methodology using confocal Raman microscopy (CRM) as a powerful sclerochronological tool for assessing preservation and identifying taphonomic alterations in the original shell carbonate. Diagenetic modifications to the shell make it challenging to visualize the internal growth patterns and typically render the specimen unusable for biogeochemical analysis (e.g., stable isotope or trace elemental analyses). The CRM mapping approach outlined here proves that CRM leads to comparable and even superior results when compared to commonly used growth increment visualization techniques on both modern and particularly on fossil shell material (A. islandica and Pygocardia rustica; both Pliocene). I also clearly demonstrate the significant impact that the recrystallization of metastable shell aragonite to calcite can have on isotopic signatures within the shell carbonate. delta18O and delta13C signatures in recrystallized shell sections of a Pliocene A. islandica (Iceland) show tremendous changes from the pristine shell. Preservation state, which is often overlooked, therefore has a substantial impact on isotope records and serious implications for environmental and climatic interpretation. I therefore strongly emphasise the crucial need for preservation assessment prior to sampling for any proxy record and that CRM is an ideal tool with which to do this. This thesis presents a reconstruction of high latitude (Svalbard) seasonality during the last warmer-than-today phase based on the archive A. islandica. Meticulously discussing all relevant considerations (e.g., shell preservation, palaeowater depth, palaeo-water chemistry, ice volume effect, etc.), I show that seasonal temperatures in a fjord setting during the Holocene Climate Optimum (HCO) were considerably higher than today (6°C on average), which impressively identifies past polar amplification (expected HCO global mean temperature 3 degree Celsius). This presents a first indication of the possible impacts of future climate change in high latitude marine regions (IPCC, 2013 stating up to 3.7 degree Celsius global temperatures until CE 2100). The further analysis of internal shell growth patterns of HCO A. islandica specimens from Svalbard detect a pronounced and significant 11-year oscillation using tools of spectral frequency analysis. Such a signal has been previously reported in other archives and has most often been associated to the 11-year solar sunspot cycle (Schwabe cycle). Solar-climate interactions remain a debated issue, as present data is temporally and spatially insufficient to decipher any hypothetical links. Therefore no previous studies propose any explanation for a mechanistic link between shell growth in biogenic marine archives and solar activity. In this thesis I propose a first possible link between solar activity and shell growth via a biological amplifying mechanism - UV radiation and phytoplankton productivity...