Ocean-Climate processes recorded in Holocene laminated sediments from the Gotland Deep, Baltic Sea
Ocean-Climate processes recorded in Holocene laminated sediments from the Gotland Deep, Baltic Sea
The Gotland Deep is an isolated deep basin within the shallow and enclosed Baltic Sea. Due to a limited exchange with oxygenated surface water with stagnant deep water, the Gotland Deep has been commonly anoxic and laminated sediments have been intermittently deposited throughout the last ~810014Cyr. Using Scanning electron microscope techniques, individual laminae, down to 50μm in thickness, with distinct mineralogical, micropalaeontological, or geochemical composition have been identified and described. Depositional laminae sequences in the form of couplets, triplets, and quadruplets of diatomaceous and lithogenic laminae are observed with a thickness range commonly between 0.45-1.25mm (mean 0.75mm). Examination of the diatom assemblages suggests that these sequences of laminae represent annual deposits, or varves. Varves are relatively uncommon and typically occur in small intervals of 2-5 varves, which are interrupted by more indistinctly laminated and homogeneous sediments.
The varves occurring in the Littorina sequence allow the placing of early diagenetic Ca-rhodochrosite laminae within the seasonal cycle as an exclusively winter-early spring deposit. This is in agreement with the seasonal distribution of major Baltic inflow events in historical records, and a direct causal link between saline inflows and Ca-rhodochrosite deposition is implied. Benthic foraminifera tests are also found to be encrusted with Ca-rhodochrosite, implying that benthic re-colonisation during oxidation events occurs concurrently with Ca-rhodochrosite formation.
Many trace metals including; Mn, Fe, Mo, U, V, Cd, Pb, Co, Ni, Cu, As and Zn, are observed to be enriched within both Ca-rhodochrosite and Fe-sulphide rich laminae. This is most likely to be caused by scavenging by particulate Fe-sulphides and Mn-oxides in the Gotland Deep, providing an effective sedimentary sink for these elements.
The relatively common occurrence of small (50-l00 μm) hexagonal y Mn-sulphide pseudomorphs is reported here for the first time. Although Mn-sulphide crystals are not usually preserved in these sediments, initial formation of Mn-sulphide may be more common than previous reports suggest. Several Mn-sulphide laminae, although heavily oxidised during sampling, are indeed directly observed within a single organic-carbon rich interval. This demonstrates that Mn-sulphide is formed and preserved, at the expense of Ca-rhodochrosite, under more reducing conditions.
University of Southampton
Burke, Ian Terence
4b21fb01-f10f-4294-bd2a-228fd83dd05c
2001
Burke, Ian Terence
4b21fb01-f10f-4294-bd2a-228fd83dd05c
Burke, Ian Terence
(2001)
Ocean-Climate processes recorded in Holocene laminated sediments from the Gotland Deep, Baltic Sea.
University of Southampton, Doctoral Thesis.
Record type:
Thesis
(Doctoral)
Abstract
The Gotland Deep is an isolated deep basin within the shallow and enclosed Baltic Sea. Due to a limited exchange with oxygenated surface water with stagnant deep water, the Gotland Deep has been commonly anoxic and laminated sediments have been intermittently deposited throughout the last ~810014Cyr. Using Scanning electron microscope techniques, individual laminae, down to 50μm in thickness, with distinct mineralogical, micropalaeontological, or geochemical composition have been identified and described. Depositional laminae sequences in the form of couplets, triplets, and quadruplets of diatomaceous and lithogenic laminae are observed with a thickness range commonly between 0.45-1.25mm (mean 0.75mm). Examination of the diatom assemblages suggests that these sequences of laminae represent annual deposits, or varves. Varves are relatively uncommon and typically occur in small intervals of 2-5 varves, which are interrupted by more indistinctly laminated and homogeneous sediments.
The varves occurring in the Littorina sequence allow the placing of early diagenetic Ca-rhodochrosite laminae within the seasonal cycle as an exclusively winter-early spring deposit. This is in agreement with the seasonal distribution of major Baltic inflow events in historical records, and a direct causal link between saline inflows and Ca-rhodochrosite deposition is implied. Benthic foraminifera tests are also found to be encrusted with Ca-rhodochrosite, implying that benthic re-colonisation during oxidation events occurs concurrently with Ca-rhodochrosite formation.
Many trace metals including; Mn, Fe, Mo, U, V, Cd, Pb, Co, Ni, Cu, As and Zn, are observed to be enriched within both Ca-rhodochrosite and Fe-sulphide rich laminae. This is most likely to be caused by scavenging by particulate Fe-sulphides and Mn-oxides in the Gotland Deep, providing an effective sedimentary sink for these elements.
The relatively common occurrence of small (50-l00 μm) hexagonal y Mn-sulphide pseudomorphs is reported here for the first time. Although Mn-sulphide crystals are not usually preserved in these sediments, initial formation of Mn-sulphide may be more common than previous reports suggest. Several Mn-sulphide laminae, although heavily oxidised during sampling, are indeed directly observed within a single organic-carbon rich interval. This demonstrates that Mn-sulphide is formed and preserved, at the expense of Ca-rhodochrosite, under more reducing conditions.
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Published date: 2001
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Local EPrints ID: 464540
URI: http://eprints.soton.ac.uk/id/eprint/464540
PURE UUID: 97667cc4-30ef-45fa-a181-619b96885a5e
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Date deposited: 04 Jul 2022 23:45
Last modified: 16 Mar 2024 19:35
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Author:
Ian Terence Burke
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