HISTORY OF BIOAVAILABLE LEAD AND IRON IN THE GREATER NORTH SEA AND ICELAND DURING THE LAST MILLENNIUM - A STORY TOLD BY BIVALVES

To identify how pollutant levels in the ocean changed through time and space requires high-resolution records of bioavailable substances prior to the era of intensified human perturbation. Here, we present the first annually resolved record of biologically available lead (Pb) and iron (Fe) in the Greater North Sea covering large portions of the time interval 1040-2004 AD. The record is based on LA-ICP-MS (Laser Ablation – Inductively Coupled Plasma – Mass Spectrometry) data from the hinge portions of twelve modern and subfossil shells of the long-lived marine bivalve Arctica islandica. Two additional specimens came from Iceland serving as a reference for a much less polluted habitat. In contrast to few previous studies on Pb/Ca and Fe/Ca in long-lived bivalve shells, this study employed the so-called LA line-scan method in order to obtain uninterrupted time-series with high temporal resolution. Prior to the industrial era, the iron content in shells from the North Sea largely remained below the detection limit. Only since ca. 1830, shell Fe levels rose gradually, strongly paralleling the increased processing and mobilization of iron in neighboring countries. Furthermore, the Fe increase likely also results from the remobilization of detrital iron near the sediment surface caused by reducing conditions that emerged after eutrophication-driven phytoplankton blooms. It is therefore likely that the combined effect of increased terrestrial runoff of iron-bearing sediments and eutrophication since the mid 19^th century accounted for a significant increase of dissolved iron in the habitat of A. islandica. Although the lead gasoline peak of the 20^th century was well recorded by the shells, bivalves that lived during the medieval heyday of metallurgy showed four-fold higher shell Pb levels than modern specimens. We hypothesize that pre-industrial bivalves consumed a larger proportion of resuspended (Pb-enriched) organics near the sea floor, whereas modern specimens preferably nurture from fresh (Pb-depleted) organics. As expected, metal loads in the shells from Iceland were much lower coincident with more pristine environmental conditions. Our study confirms that shells of long-lived bivalves provide a powerful tool for retrospective environmental biomonitoring.