Sclerochronology and modelling: combining annually resolved bivalve records and biogeochemical models to understand the shelf seas

Abstract

The shelf seas are extremely vulnerable to the effects of climate change. Projected to warm at substantially greater rates than the open ocean, understanding how shelf sea systems operate and how they will respond to future change is of vital importance. Policy decision-makers rely on high quality information to ensure the protection of marine habitats and ecosystem services. While model studies can provide such data, they require spatially and temporally-extensive datasets for verification. Currently, this type of data is highly limited for the shelf seas, particularly at depth.

Sclerochronology of bivalve molluscs has shown great potential to extend instrumental data for the shelf seas, providing absolutely-dated, multi-centennial, annually-resolved archives of past ocean environment, analogous to dendrochronology in terrestrial environments. Bivalve molluscs have a wide distribution, and can be found on the shelf environments living at depth on the sea floor. Yet sclerochronology is a developing field with a number of fundamental research gaps limiting the use of sclerochronology as a valuable marine proxy. This thesis addresses three of those gaps:

Chapter 2 (Methods Development: Imaging shells for sclerochronology) presents a novel and non-destructive method for imaging the internal growth bands of bivalves used in sclerochronology – micro computed tomography (micro-CT). Micro-CT uses x-rays to create 3D high resolution images of the internal structure of specimens. Experiments evaluated whether density or resolution could limit bivalves from being imaged and showed that subtle microstructural features can be seen in the micro-CT images of even the most dense bivalve species. The research suggested that with future development in micro-CT technology including increased resolution and power, analysis of bivalve growth bands via micro-CT may be possible, significantly reducing the time required to produce highly valuable sclerochronology records and allowing more records to be constructed.

Chapter 3 (Sclerochronology and 1D modelling: a novel study using a 1D ecosystem model to better interpret sclerochronology records) focuses on a fundamental problem in sclerochronology - the lack of understanding regarding the mechanistic drivers of shell growth. Here, a 1D ecosystem model GOTM-ERSEM- BFM was used explore these mechanisms. The model was able to simulate the shell growth of a central North Sea composite chronology which allowed exploration of ecosystem processes to understand the mechanisms that lead to growth. Experiments manipulating the meteorological inputs to the model mechanistically attributed variability in surface heating and wind temperature as key controls on shell growth in the central North Sea.

Chapter 4 (Southern North Sea sclerochronologies: using shell records to test hypotheses across hydrological and biological gradients) addresses the scarcity of long-term productivity information in the North Sea which currently limits an understanding of historical variability. In particular, accurate measurements of North Sea productivity are limited by the poor quantification of sub-surface chlorophyll which cannot be measured by remote sensing or surface phytoplankton surveying. 12 new sclerochronology records of the bivalve mollusc Arctica islandica were produced from 4 locations in the southern North Sea to test if bivalve growth differs across productivity and hydrography gradients, and investigate whether shell records capture sub-surface chlorophyll variability. Differences in the rate of raw growth of Arctica islandica was demonstrated between regions of high and low productivity, supported by variations in synchronicity (estimated population signals) of multiple composite chronologies constructed using the shells records. These results suggest that sub surface chlorophyll may play a role in Arctica islandica shell growth but how this interacts with stratification regimes in the North Sea is difficult to quantify due to complex interactions of biology and other hydrographical features in the region. Further research is required in other locations to better understand impact of stratification and sub-surface productivity on shell growth and subsequently North Sea ecosystems.

The novel research presented in this thesis has advanced the fields of sclerochronology and ecosystem modelling and has laid the foundation for transformative approaches to sclerochronology. By developing the methods needed to understand long-term variability of shelf sea environments, particularly at depth, this work has contributed to improving the understanding of past and future climate change in the shelf seas.