Jivcov, Sandra
(2019).
From Lake to Ocean: Disentangling past and present depositional processes and environmental conditions of sub-Antarctic South Georgia using molecular and isotopic tools.
PhD thesis, Universität zu Köln.
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Abstract
The sub-Antarctic is a key region for the understanding of the coupling of Northern and Southern Hemispheres’ reactions to climate change. Until now, only few marine sedimentary records from this region were investigated because they are often lacking in eligible mate- rial for radiocarbon dating. Here, this topical problem is addressed by compound-specific radiocarbon analysis (CSRA) of biomarkers, derived from sedimentary records of South Georgia. Intrinsic characteristics of different biomarkers as well as depositional processes were investigated, in order to identify suitable compounds for the development of confident sediment chronologies. Environmental changes in the study area were traced on the basis of comprehensive multi-proxy analyses of marine and lacustrine sedimentary records. Radiocarbon ages of n-fatty acids, n-alkanes, n-alcohols, total organic carbon (TOC) and macrofossils of a coastal marine sediment core (Co1305) of Little Jason Lagoon and a lacus- trine sediment core (Co1308) of Allen Lake A were measured, in order to identify sources of the organic matter and to asses timescales of sediment transport. Similar temporal offsets of c. 1,900 to 2,600 years between production and deposition of co-occurring plant macrofos- sils and high molecular weight (HMW) n-fatty acids and n-alkanes point to common origins and transport pathways of these different sample types. Preservation and retention of the organic matter on land seems to be favored by climatic conditions. Sedimentary bedrock is another source of land-derived organic carbon (OC) in the study area. Petrogenic OC is commonly free of 14C and therefore influences radiocarbon ages of bulk sediments considerably. In order to quantify the contributions of OC from petro- genic, terrigenous (plants and soils) and marine sources to the surface sediments of different aquatic environments (a lake, a marine inlet, two fjords and an off-shore site), three end- member mass balance calculations were successfully applied. A clear spatial trend in the proportions of ancient OC in the sediments indicates that export of petrogenic carbon is mainly controlled by the activity of glaciers in the supply area. Correlation of ice-rafted debris (IRD) with high TOC ages of the coastal marine downcore record Co1305 supports that mechanical erosion of the bedrock by glaciers most efficiently mobilizes ancient OC. Because of the influence of differently "aged" material from various sources on land, ra- diocarbon ages of terrigenous components or of TOC cannot be used for a chronological purpose in the coastal marine setting of Little Jason Lagoon. CSRA revealed that 14C ages of mainly marine biomarkers (n-C16 fatty acids and n-C22 alcohols) likely yield more conf- ident approximations of the time of sediment formation at this site. Proportions of terrige- nous homologues, contributing to sedimentary pools of n-C16 fatty acids and n-C22 alcohols, seem to be related to the individual diagenetical behaviors of the respective biomarkers. 14C ages of co-occurring n-C16 fatty acids and n-C22 alcohols indicate that post-depositional pro- cesses change proportions of marine and terrigenous homologues of n-C16 fatty acids to a higher extend than of n-C22 alcohols. It seems therefore advantageous to combine n-C16 fatty acid ages along with n-C22 alcohol ages to improve the accuracy of the marine sedi- ment chronology, particularly for the lower part of the sedimentary record Co1305. Sedimentation is assumed to have started c. 10,000 cal BP in the setting of Little Jason Lagoon. The record Co1308 of Allen Lake A is going back to c. 7,500 cal BP. A marked environmental change was encountered in both records to have happened around 4,000 cal BP, when climatic conditions ameliorated after a cold phase. A marked vegetational change at this time in the setting of Allen Lake A seems to have caused several sedimentological and geochemical changes in the lacustrine record. Where sedimentary proxies of the lacustrine record Co1308 reveal a long term trend of successively ameliorating climatic conditions since c. 7,500 cal BP, various environmental changes, including glacier fluctuations and variations in marine and terrigenous productivity, are indicated in the record of Little Jason Lagoon. Combination of comprehensive information of both sedimentary records enabled the iden- tification of system dependent variability of the different aquatic environments as well as environmental transitions of a likely regional extend.
| Item Type: | Thesis (PhD thesis) | ||||||||
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| URN: | urn:nbn:de:hbz:38-94780 | ||||||||
| Date: | 26 March 2019 | ||||||||
| Language: | English | ||||||||
| Faculty: | Faculty of Mathematics and Natural Sciences | ||||||||
| Divisions: | Faculty of Mathematics and Natural Sciences > Department of Geosciences > Institute of Geology and Mineralog | ||||||||
| Subjects: | Natural sciences and mathematics Earth sciences |
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| Date of oral exam: | 13 June 2018 | ||||||||
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| Funders: | German Research Foundation (DFG) | ||||||||
| Projects: | Schwerpunktprogramm SSP 1158 Antarktisforschung | ||||||||
| Refereed: | Yes | ||||||||
| URI: | http://kups.ub.uni-koeln.de/id/eprint/9478 |
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