Anomalous 18O-depletions of Madeiran Glycymeris growth increments - A new tool to trace meridional shifts of the Azores Front

Description

Despite the positive results obtained using saltwater clam Glycymeris spp. for palaeoenvironmental reconstructions in the extratropical North Atlantic domain, its potential has not been investigated in the subtropical North Atlantic region. The aim of this study was to investigate cockles (G. vanhengtsumi) as archives for climate perturbations from the subtropical region by measuring the oxygen isotope compositions from the annual growth increments. The shells’ habitat ranging from 150-160 m depth offshore Madeira is exposed to relatively minor seasonal contrasts in terms of temperature compared to higher latitudes. The region itself, however, is exposed to the highly dynamic system of the Azores Current and its associated front on the north-eastern boundary of the North Atlantic Subtropical Gyre.The three analysed shells were collected (one alive and two as sub-fossil specimens) near the Desertas Islands, Madeira in 2014. They were selected from a set of synchronized increment series of a robust site chronology for the 1950–2012 period which was used to explore shell growth responses to environmental parameters in an earlier study. As the three increments series are overlapping in time, the collected carbonate samples cover the period between ~1957 and 2010. Annual increments were sampled at seasonal resolution (min. 3 max 7 sample points per increment) using a computer-controlled micromill device (ESI New Wave Research, Portland). Stable carbon (δ13C) and oxygen (δ18O) isotope compositions of the carbonate samples were analysed by an IRMS. Oxygen isotopic ratios measured from the shells’ carbonate and regional seawater δ18O were used to calculate water temperatures on a seasonal resolution based on isotope equilibrium relationships. The results mostly coincided with the expected seasonal temperature range (17-19°C) of the shells habitat, except from a few anomalously high temperature values inferred (20-23°C), e.g. for 1986-1988 and in the late 70’s. During these years, the modelled position of the Azores Front shifted southward unexpectedly (~31° N) compared to its usual position situated between 33° and 35° N. This relationship suggests that anomalous δ18O-depletions of the Madeiran cockle increments are not necessarily connected to real water temperature anomalies as the upper layer of the water column has prominently lower δ18O values north of the Azores Front than south of it. Our results suggest that sclerochronological analysis of Glycymeris spp. shells combined with high-resolution stable oxygen isotope measurements could be used to track regional changes of this subtropical current system.

Period	17 Jun 2019
Held at	5th International Sclerochronology Conference (ISC 2019)
Event type	Conference
Location	Split, Croatia
Degree of Recognition	International