Continuous marine radiocarbon reservoir calibration and the ¹³C Suess effect in the Irish Sea: Results from the first multi-centennial shell-based marine master chronology

Abstract

The identification in various proxy records of periods of rapid (decadal scale) climate change over recent millennia, together with the possibility that feedback mechanisms may amplify climate system responses to increasing atmospheric CO₂, highlights the importance of a detailed understanding, at high spatial and temporal resolutions, of forcings and feedbacks within the system. Such an understanding has hitherto been limited because the temperate marine environment has lacked an absolute timescale of the kind provided by tree-rings for the terrestrial environment and by corals for the tropical marine environment. Here we present the first annually resolved, multi-centennial (489-year), absolutely dated, shell-based marine master chronology. The chronology has been constructed by detrending and averaging annual growth increment widths in the shells of multiple specimens of the very long-lived bivalve mollusc Arctica islandica, collected from sites to the south and west of the Isle of Man in the Irish Sea. The strength of the common environmental signal expressed in the chronology is fully comparable with equivalent statistics for tree-ring chronologies. Analysis of the ¹⁴C signal in the shells shows no trend in the marine radiocarbon reservoir correction (ΔR), although it may be more variable before ∼ 1750. The δ¹³C signal shows a very significant (R² = 0.456, p < 0.0001) trend due to the ¹³C Suess effect.

Introduction

It has been suggested that the periodic growth increments in the shells of long-lived bivalve molluscs might be used to construct an absolute chronology (Briffa, 1995) for the marine environment of the kind that tree-rings have provided for the terrestrial environment (Clark, 1968, Thompson and Jones, 1977, Richardson, 2001). By using the dendrochronological technique of crossdating (Fritts, 1976 pp 20–23; Briffa, 1995), such a chronology could in principle be extended back in time well beyond the lifetime of any living animal. In particular, the infaunal benthic bivalve Arctica islandica has been shown to fulfil the four criteria outlined by Thompson and Jones (1977) for use in a shell-based chronology: synchronized growth, annual banding, longevity and growth throughout life (Witbaard et al., 1997). Nevertheless, only two published studies have so far successfully crossdated from live caught animals to fossil shells (Marchitto et al., 2000, Schöne et al., 2003), and one other study (Scourse et al., 2006) has cross-matched three ¹⁴C dated shells from the early part of the last millennium to form a floating chronology which has not been linked to any specimens of known age. The lengths of these chronologies are 154 years, 136 years and 287 years respectively. Here we present a 489-year (AD 1516–2004) master chronology which has been constructed using shells of A. islandica collected from Irish Sea waters off the west coast of the Isle of Man. This is not only the longest cross-matched shell-based chronology constructed so far; it also represents a significant advance in terms of the number of crossdated shells used (fifty eight time series from forty two individual animals), the persistence of sample depth (at least two shells back to AD 1525) and the persistence of a strong common signal (the expressed population signal (Wigley et al., 1984)) exceeds 0.83 throughout). In addition this is the first shell-based chronology to use crossdating to determine the calendar dates of ¹⁴C dated fossil shells from animals whose lifetimes do not overlap with those of any live collected animal.

A. islandica has significant potential as a proxy recorder of the marine environment of the shelf seas adjacent to the North Atlantic. Annual growth increment widths of juvenile A. islandica in the North Sea have been shown to be positively related to primary productivity (Witbaard et al., 1999). Positive relationships between shell growth and δ¹⁸O-derived bottom water temperature have been shown for A. islandica from Iceland (Schöne et al., 2005a) and the North Sea (Schöne et al., 2005b). Increment widths may also be affected by the length of the growing season and the timing of formation of the growth line, which are also likely to be influenced by bottom water temperature (Weidman et al., 1994). In addition, links have been made between the variability of the increment widths and changes in the North Atlantic Oscillation (NAO) index (Schöne et al., 2003, Helama et al., 2007), and in the hydrography of the northern North Sea (Witbaard et al., 1997).

Proxy records are also available in the geochemistry of the aragonitic shell material. Oxygen isotope ratios measured from samples taken across adjacent growth increments from A. islandica show a clear seasonal signal (Weidman et al., 1994, Witbaard et al., 1994, Schöne et al., 2004, Schöne et al., 2005a) indicating that they are linked to bottom seawater temperatures. Shell δ¹⁸O appears to be in equilibrium with the ambient seawater (Weidman et al., 1994) and this relationship has been used to attribute a mass mortality of tilefish in the Mid Atlantic Bight in 1882 to an extreme cold seawater event (Marsh et al., 1999). The relationship between shell δ¹⁸O in A. islandica and seawater temperature in the Gulf of Maine has been used to infer changes through the last millennium in the dynamics of the Labrador Current and Gulf Stream (Wanamaker et al., 2008a). Changes in ¹⁴C concentrations in the shell of a specimen of A. islandica from Georges Bank have been used to study the history of the radiocarbon bomb pulse in the region and to indicate changes in the formation of deep water in the Labrador Sea during the 1960s and 1970s (Weidman and Jones, 1993). More recently, changes in ¹⁴C concentrations in cross-matched specimens from the north Icelandic shelf have been used to document the history of the marine radiocarbon reservoir, and hence probable changes in the position of the polar front, through the last millennium (Wanamaker et al., 2008b).

The potential of shells from Isle of Man waters to be used as a proxy for changing environmental conditions is enhanced because the A. islandica populations are very close to two stations where marine environmental data has been monitored over several decades by Port Erin Marine Laboratory, University of Liverpool (Evans et al., 2003). Oxygen, nutrients, chlorophyll, salinity and sea surface temperature (SST) have been routinely measured at the Cypris station (Fig. 1; position 54° 5.50′N 4° 50.0′W) for periods of up to 54 years. In addition, one of the longest continuous records of SST in existence is the series measured on a daily basis at the Port Erin breakwater (Fig. 1; position 54° 5.40′N 4° 46.50′W) since 1904. These instrumental data series provide a basis for the robust calibration of the growth response of the shells to local environmental variables, and the potential for such a calibration to be extended to the interpretation of A. islandica growth increment patterns across other parts of its range.

A significant benefit of multi-centennial shell-based chronologies is the potential to monitor at high resolution changes in the marine radiocarbon reservoir. Carbon taken up by marine organisms is deficient in ¹⁴C when compared with carbon incorporated into contemporaneous terrestrial organisms. This deficiency is a function of the deep ocean circulation and the hydrographic connections between the deep oceans and shelf seas (Stuiver et al., 1986). Where a site has been affected at different periods by water masses of different ages, these changes should be reflected in corresponding variations in the concentration of ¹⁴C in shell material, and if the calendar age of the shell material is known, a history of regional hydrography can be reconstructed (Eiríksson et al., 2004, Ascough et al., 2006). Changes in the strength and position of water masses may act as significant amplifiers in the climate system (Dickson et al., 2003, Hald et al., 2007) and a detailed knowledge of their dynamics significantly strengthens the predictive power of climate models.

In the research presented here, the absolutely dated chronology has been used to time constrain ¹⁴C and δ¹³C determinations for the shell material, allowing the reconstruction of a multi-centennial, high resolution history of changes in the marine radiocarbon reservoir effect and in the marine ¹³C Suess effect. A. islandica is common in the coastal shelf seas of the North Atlantic, both at the present time and throughout the Holocene (Dahlgren et al., 2000); the results of this research therefore demonstrate the feasibility of an integrated recent history of the hydrography of this climatically important region.