Cold-Water-Carbonates in the Bay of Biscay.

Boxleitner, Max (2013) Cold-Water-Carbonates in the Bay of Biscay. (Master thesis), Saint Petersburg State University ; University of Hamburg, Saint Petersburg, Russia ; Hamburg, 104 pp.

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01 Element and stable isotope ratios.xlsx - Supplemental Material
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02 JCP measurements.xlsx - Supplemental Material
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03 U-Th preliminary ages.xlsx - Supplemental Material
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Abstract

Marine carbonates are widely used archives for paleoceanographic and climatic reconstructions. The cold-water carbonates investigated in this study are skeletal remains of the two cold-water coral (CWC) species Lophelia pertusa and Madrepora oculata. These framework-building scleractinian coral species are widespread on the continental slopes of the world’s oceans and their aragonitic skeletons can be precisely dated with radiometric techniques, which makes them an interesting archive. In the northeast Atlantic CWCs are known to build different ecosystems. While reefs in Norway, mound structures off the coast of Ireland and CWC occurrences in the Gulf of Cadiz are relatively well studied, comparably little is known about the Bay of Biscay, a region that connects the eastern temperate Atlantic with the more northern regions. In order to examine the CWC distribution and its history in the Bay of Biscay, this study analyzes coral samples from six sediment cores that were retrieved during METEOR Cruise M84/5 in June 2011 along a depth transect (539mbsl - 980mbsl) in the St. Nazaire canyon in the northern Bay of Biscay. In this setting CWCs currently occur in patches and thickets and not in large reefs like they are e.g. known from the Norwegian coast.
The aim of tthis study is to examine the CWC growth in the St. Nazaire canyon, determine how long CWCs have been growing in this setting and reconstruct under which environmental conditions they lived.
For this purpose the coral samples were mechanically and chemically cleaned, dated via the U/Th-decay chain and analyzed for their skeletal stable isotope and element ratios. The measurement of these ratios was carried out on a Quadrupole Inductively Coupled Plasma Mass Spectrometer (Q ICP-MS).
The dating of the samples revealed coral ages from 12,5 kyBP to present-day ages. This shows that since the end of the last glacial the St. Nazaire canyon represented a suitable setting for CWC growth. While other CWC ecosystems further south, e.g. in the Gulf of Cadiz, seem to have thrived only during glacial times, the dated coral samples of this study indicate, that the St. Nazaire canyon rather belongs to the CWC ecosystems that flourish during interglacial times like in regions further north. Moreover, the dating of the coral samples documents numerous inversions in the sediment cores that complicate the reconstruction of the deposition history, but at the same time emphasize the special geological character of the canyon setting.
Given the time frame, some of the measured element ratios could be used as proxies to reconstruct environmental parameters that characterized the CWC ecosystem in former times: Paleotemperature reconstruction, based on a Mg/Li-proxy calibrated for L. pertusa, showed that seawater temperatures throughout the Holocene were in the range of ~9-10°C. Similarly stable were the pH-conditions during the Holocene. The pH-values were reconstructed by applying a U/Ca-proxy that has only recently been defined and so far has not been used for fossil corals. The pH-values during the last 12,5 ky fluctuated slightly (±0,2) around the modern pH-value of 8. In contrast to pH and temperature, coral Ba/Ca ratios, interpreted as an indicator for paleoproductivity, showed an almost linear decrease of around 40%. The underlying reasons could include changes in the current regime and/or changes in the surface productivity, probably triggered by decreasing terrigenous input. This could be one of the factors, why current CWC growth only occurs relatively sparsely.
Physical and geochemical boundary conditions of CWC growth are a topic of ongoing research. The reconstructed environmental parameters are important factors, but only in the context with other parameters like current velocities and sedimentation rates that influence and constrain CWC distribution too. Hence, along with others, the reconstructed factors temperature, pH and nutrient supply seem to create a generally favorable setting for CWC growth in the St. Nazaire canyon since ~12,5 kyBP.

Document Type: Thesis (Master thesis)
Thesis Advisors: Dullo, Wolf-Christian, Liebetrau, Volker and Kuznetsov, Vladislav Yurievich
Keywords: Cold-water carbonates; Cold-water corals; Lophelia pertusa; Madrepora oculata; Atlantic, Northeast; Bay of Biscay, St. Nazaire canyon; Environmental conditions; Ecosystem; Seawater temperature; pH and nutrient supply; Holocene
Research affiliation: OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-MG Marine Geosystems
OceanRep > GEOMAR > FB1 Ocean Circulation and Climate Dynamics > FB1-P-OZ Paleo-Oceanography
Projects: POMOR
Expeditions/Models/Experiments:
Date Deposited: 16 Mar 2015 10:45
Last Modified: 16 Mar 2015 10:45
URI: http://oceanrep.geomar.de/id/eprint/28039

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