Variations in sediment provenance during the past 3000 years off the Tagus River, Portugal

Alt-Epping, U., Stuut, J. B. W., Hebbeln, D. and Schneider, Ralph (2009) Variations in sediment provenance during the past 3000 years off the Tagus River, Portugal Marine Geology, 261 (1-4). pp. 82-91. DOI 10.1016/j.margeo.2008.11.008.

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Marine sediments from the Portuguese shelf are influenced by environmental changes in the surrounding continental and marine environment. These are largely controlled by the North Atlantic Oscillation, but additional impacts may arise from episodic tsunamis. In order to investigate these influences, a high resolution multi-proxy study has been carried out on a 5.4 m long gravity core and five box cores from the Tagus prodelta on the western Portuguese margin, incorporating geochemical (C-org/N-total ratios, delta C-13(org) delta N-15, delta O-18, C-org and CaCO3 content) and physical sediment properties (magnetic susceptibility, grain-size). Subsurface data of the five box cores indicate no major effect of early postdepositional alteration. Surface data show a higher fraction of terrigenous organic material close to the river mouth and in the southern prodelta. Gravity core GeoB 8903 covers the last 3.2 kyrs with a temporal resolution of at least 0.1 cm/yr. Very high sedimentation rates between 69 and 140 cm core depth indicate a possible disturbance of the record by the AD1755 tsunami, although no evidence for a disturbance is observed in the data. Sea surface temperature and salinity on the prodelta, the local budget of marine NO3- as well as the provenance of organic matter remained virtually constant during the past 3.2 kyrs.A positive correlation between magnetic susceptibility (MS) and North Atlantic Oscillation (NAO) is evident for the past 250 years, coinciding with a negative correlation between mean grain-size and NAO. This is assigned to a constant riverine supply of fine material with high MS, which is diluted by the riverine input of a coarser, low-MS component during NAO negative, high-precipitation phases. End-member modelling of the lithic grain-size spectrum supports this, revealing a third, coarse lithic component. The high abundance of this coarse end-member prior to 2 kyr BP is interpreted as the result of stronger bottom currents, concentrating the coarse sediment fraction by winnowing. As continental climate was more and prior to 2 kyr BP (Subboreal), the coarse end-member may also consist of dust from local sources. A decrease in grain-size and CaCO3 content after 2 kyr BP is interpreted as a result of decreasing wind strength. The onset of a fining trend and a further decrease in CaCO3 around AD900 occurs simultaneous to climatic variations, reconstructed from eastern North Atlantic records. A strong increase in MS between AD1400 and AD1500 indicates higher lithic terrigenous input, caused by deforestation in the hinterland. (C) 2008 Elsevier B.V. All rights reserved.

Document Type: Article
Additional Information: Sp. Iss. SI461YY
Keywords: organic matter provenancenutrient budget grain size tsunami nao portuguese margin tagus river north-atlantic oscillation last 2000 years climatic variability size distributions iberian peninsula marine-sediments stable-isotopes nitrogen sea matter
Research affiliation: Kiel University
OceanRep > The Future Ocean - Cluster of Excellence
Refereed: Yes
DOI etc.: 10.1016/j.margeo.2008.11.008
ISSN: 0025-3227
Projects: Future Ocean
Date Deposited: 11 Feb 2011 12:14
Last Modified: 04 Aug 2017 12:15

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