Postglazialer Anstieg des Meeresspiegels, Paläoklima und Hydrographie, aufgezeichnet in Sedimenten der Bermuda inshore waters
Postglacial rise of sea level, palaeoclimate and hydrography, recorded in sediments of the Bermuda inshore waters
by Rüdiger Dr. Vollbrecht
Date of Examination:1997-01-13
Date of issue:2000-10-26
Advisor:Prof. Dr. Dieter Meischner
Referee:Prof. Dr. Jochen Hoefs
Referee:Prof. Dr. Dieter Meischner
Referee:Prof. Dr. Walter Riegel
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Abstract
English
The principal aim of this study was to clarify the sedimentary history of the Bermuda inshore waters during postglacial time. From this, clues to the deglacial history of regional sea-level and palaeoclimate were expected.Fourty-six kilometres of UNIBOOM seismic profiles and 53 sediment cores were studied from various locations in the major inshore waters: Castle Harbour, Great Sound, Harrington Sound, Port Royal Bay, St. George's Harbour. Of these, Harrington Sound was studied most intensely.Aside from core description techniques, the following methods were employed: X-ray radiography, warve counting, optical and scanning electron microscopy, coarse fraction analysis, micropalaeontological techniques, X-ray diffraction, geochemical techniques (bulk carbonate, CHN-analyses, GC-MS, stable oxygen and carbon isotopes), and radiocarbon dating (conventional, AMS).The Holocene sedimentary succession in Harrington Sound comprises 15 limnic, brackish, and marine facies. Above the Pleistocene soil or carbonates, a basal Holocene peat is usually developed. This basal peat accumulated in freshwater marshes very close to the ambient sea-level. Radiocarbon age-depth data on peat from various elevations give a measure of deglacial sea-level rise during the last 10,400 14C-years BP (~12,500 sidereal years BP). Comparison with another glacioeustatic sea-level curve, i.e. that of Barbados, reveals significant differences the reasons of which are still under discussion. Previously unrecognized sources of error include: (1) tectonic-isostatic movements during global deglaciation (possible for both settings, but more probable for Barbados), and (2) downslope transport of Acropora palmata debris (Barbados).During the last 10,400 14C-years BP, sea level rose from below -35 m to present datum. In response to this, the formerly subaerial inshore basins became flooded, at first by fresh water lenses buoyant at sea level. In the deep depressions of Harrington Sound, the Holocene limnic succession starts with peat and intervening lacustrine chalk. This reflects a freshwater marsh, with small ponds, established in response to the rising sea level. Deposition of lake marl commenced throughout much of the Harrington Sound lake when the continuously rising lake level flooded former marsh floors. Continued rise of sea level led to encroachment of marine pore water through the karstified bottom of Harrington Sound, establishing meromictic conditions in the deeps and brackish conditions in the shallows by about 8.000 yr BP. Limnic mollusc and ostracod species became quickly replaced by euryhaline ones. About 1,5 kyr later, holomixis resumed throughout the basin, and restricted marine conditions were established.The last major environmental change took place a few hundreds of years ago when, possibly in historic time already, Harrington Sound acquired a surficial connection to the adjacent lagoon. In response to the improved water exchange, a different suite of marine fauna including the branching coral Oculina entered the basin and started to re-shape the inshore-water bottom. On principle, each of the inshore basins ran through the same history of emersion, flooding, ingression of sea-water, and deepening. Modifications to the general scheme resulted from differences in: (1) the degree of isolation from the surrounding ocean, (2) antecedent topography, (3) hydrography, (4) erosion, (5) autotrophic communities, and (6) reef communities.
Keywords: postglacial; sea level; palaeoclimate; paleoclimate; hydrography; Bermuda; sediments; Pleistocene; Holocene; Quaternary; inshore waters; lakes; laminites; solar cycles; lacustrine; marine; geology; carbonates; soil; karst; peat; isotope geochemistry; radiocarbon dating; facies; palaeoecology; paleoecology
Schlagwörter: Postglazial; Meeresspiegel; Paläoklima; Hydrography; Bermuda; Sedimente; Pleistozän; Holozän; Quartär; Seen; Küste; Laminite; Solarzyklen; Sonnenzyklen; marin; lakustrin; Geologie; Karbonate; Böden; Karst; Torf; Isotopen-Geochemie; Altersbestimmung; Fazies; Paläökologie