Provenance of organic matter and nutrient conditions on a river- and upwelling influenced shelf: A case study from the Portuguese Margin

Abstract

Organic matter contained in surface sediments from four regions on the western Portuguese shelf, which are influenced by coastal upwelling and fluvial input, was analysed with respect to elemental organic carbon (C org) and nitrogen (N total) content and isotopic carbon and nitrogen ratios ( δ 13C org, δ 15N). C org/N total weight ratios and δ 13C org values are interpreted in terms of terrigenous or marine organic matter sources, supported by CaCO 3 content. Organic matter in the shelf sediments is mainly of marine origin, with increasing terrigenous components only close to rivers and estuaries. In the northern shelf region the data indicates significant terrigenous supply by the Douro River. North of the Nazaré Canyon organic matter composition implies a mainly marine origin, with a higher terrestrial influence close to the canyon head. Organic matter composition in the central shelf region, which is dominated by the Tagus Estuary and the Tagus prodelta, reveals a change from a continental-type signature within the estuary to a more marine-type signature further to the west and south of the estuary mouth. In the southern region near Cape Sines the geochemical properties clearly reflect the marine origin of sedimentary organic matter. Sedimentary δ 15N values are interpreted to reflect various degrees of assimilation of seasonally upwelled nitrate, in relation to the upwelling centres. In the estuarine environment, inputs of agriculturally influenced dissolved inorganic nitrogen are reflected in the sediments. No evidence for N 2-fixation or denitrification is found. On the central shelf north of the Nazaré canyon, sedimentary δ 15N values are close to marine δ 15NO 3 − and thus indicate a complete NO 3 − assimilation and N-limitation of marine production. Light δ 15N values in distal sediments off the Douro River mouth and in samples south of C. Sines reflect high NO 3 − supply and a close proximity to the seasonal upwelling centres. Particularly in sediments form the Sines region, light δ 15N values in southern samples reflect stronger upwelling further south.