THE ORGANO-GEOCHEMICAL RECORD FROM THE ANDFJORDEN, NORTHERN NORWAY: PALEOPRODUCTIVITY DURING THE LATE GLACIAL ÑEARLY HOLOCENE
KNIES, JOCHEN . Geological Survey of Norway, Trondheim, Norway.
Hald, Morten . University of Tromsù, Tromsù, Norway.
Ebbesen, Hanne . University of Tromsù, Tromsù, Norway.
In this paper, we investigated the organic geochemical record from the Andfjorden, northern Norway, emphasising particularly on the rapid transition during the Bølling/Allerød - Younger Dryas - Early Holocene climate oscillations. The age model of sediment core JM99-1200 (69°15.95 N, 16°25.09 E; 476 mbsf; ~11 m core length) is based on six AMS 14C datings done on macrofossils. The record shows an undisturbed sequence with sedimentation rates up to 634 cm/kyr. We sampled the record every 5 cm revealing a resolution of up to ~10 years/sample.
Technically, we performed total organic carbon (TOC), carbonate (CaCO3) and biogenic opal analyses. We looked at the organic carbon composition by studying the stable isotope composition of TOC and nitrogen, the TOC/nitrogen ratio, the kerogen types by Rock Eval pyrolysis, and various biomarkers indicating the supply of marine, and terrigenous organic matter. We calculated accumulation rates based on linear sedimentation rates and dry bulk density data and terrigenous free relative percentages of the biogenic components.
The perfect fit between carbonate weight percent and planktic forams/g Sediment from nearby record T-79-51/2 (R2= 0.92) (Hagen 1995) suggests a biogenic origin of the carbonate fraction in JM99-1200. Variable sedimentation rates from ~46 to 634 cm/kyr, however, modulated significantly the carbonate record. In contrast, calculations of terrigenous free relative percentages show two distinct carbonate maxima, one before (Allerød) and one after (Early Holocene) the Younger Dryas cold period. The overall good correlation (R2= 0.8) between the terrigenous free relative percentages of biogenic carbonate and the 13C values of the organic matter led to the assumption that the marine organic matter input is closely coupled to the biogenic carbonate production. The TOC record seems not to be influenced by variable sedimentation rates as the CaCO3 record does. Instead, three distinct TOC minima are coupled with changes in the grain size distribution, i.e. increased sand input. According to Canfield (1994), constant preservation of organic carbon at accumulation rates greater than 0.04 g cm-2 yr-1 is suggested. However, we cannot exclude a distinct clastic dilution effect at sedimentation rates up to ~600 cm/kyr. In order to avoid modulation of paleoproductivity estimations by applying a constant linear sedimentation rate model, we calculated the relative amounts of marine and terrigenous organic matter input by using a simple two endmember mixing model based on stable carbon isotopes of the organic matter. We defined a marine endmember value of -20.3 ‰ and used a generally accepted terrigenous endmember for the study area of -27 ‰. The results indicate a predominance of marine organic matter input throughout the record (up to 80 % during the Holocene Climate Optimum). Freshwater supply is rather limited as indicated by subordinate concentrations of freshwater dinocysts (e.g. Pediastrum) (K. Grøsfjeld, pers. comm.) and does not modulate the record. Despite variable sedimentation rates and possible dilution effects, we suggest that the relative amounts of marine organic carbon can be used as surface water productivity indicator in the Andfjorden. Characteristic cold periods, like the Older Dryas, the Intra-Allerød Cold Period (IACP) and the 8.200 cold event are marked by a 30 % drop in marine organic carbon input and a parallel increase in terrestrial organic matter supply due to enhanced glacial fluctuations.
Canfield, D.E., 1994, Factors influencing organic carbon preservation in marine sediments: Chemical Geology, v. 114, p. 315-329.
Hagen, S., 1995, Watermass characteristics and climate in the Nordic sea during the last 10.200 years: Unpublished Master Thesis, Institute of Biology and Geology, University of Tromsù, Norway, pp. 100
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