Carbonate preservation in Pliocene to Holocene periplatform sediments (Great Bahama Bank, Florida Straits) [Elektronische Ressource] / vorgelegt von Johanna Schwarz

Haake

Informations

• Publié par: universitat_bremen
• Publié le: 01 janvier 2007
• Nombre de lectures: 21
• Langue: English
• Poids de l'ouvrage: 2 Mo

Extrait

Carbonate preservation in Pliocene to Holocene
periplatform sediments
(Great Bahama Bank, Florida Straits)
Dissertation
zur Erlangung des Doktorgrades
der Naturwissenschaften
am Fachbereich Geowissenschaften
der Universität Bremen
vorgelegt von
Johanna Schwarz
Bremen, 2007 Tag des Kolloquiums:
16. Mai 2007
Gutachter:
Rebecca Rendle-Bühring
Hildegard Westphal
Prüfer:
Gerhard Bohrmann
John Reijmer
IIIIIIVAbstract
Abstract
The oceanic carbon cycle mainly comprises the production and dissolution/
preservation of carbonate particles in the water column or within the sediment. Carbon
dioxide is one of the major controlling factors for the production and dissolution of carbonate.
There is a steady exchange between the ocean and atmosphere in order to achieve an
equilibrium of CO ; an anthropogenic rise of CO in the atmosphere would therefore also 2 2
increase the amount of CO in the ocean. The increased amount of CO in the ocean, due to 2 2
increasing CO -emissions into the atmosphere since the industrial revolution, has been 2
interpreted as “ocean acidification” (Caldeira and Wickett, 2003). Its alarming effects, such as
dissolution and reduced CaCO formation, on reefs and other carbonate shell producing 3
organisms form the topic of current discussions (Kolbert, 2006).
Decreasing temperatures and increasing pressure and CO enhance the dissolution of 2
carbonate particles at the sediment-water interface in the deep sea. Moreover, dissolution
processes are dependent of the saturation state of the surrounding water with respect to calcite
or aragonite. Significantly increased dissolution has been observed below the aragonite or
calcite chemical lysocline; below the aragonite compensation depth (ACD), or calcite
compensation depth (CCD), all aragonite or calcite particles, respectively, are dissolved.
Aragonite, which is more prone to dissolution than calcite, features a shallower lysocline and
compensation depth than calcite. In the 1980´s it was suggested that significant dissolution
also occurs in the water column or at the sediment-water interface above the lysocline.
Unknown quantities of carbonate produced at the sea surface, would be dissolved due to this
process. This would affect the calculation of the carbonate production and the entire carbonate
budget of the world´s ocean. Following this assumption, a number of studies have been
carried out to monitor supralysoclinal dissolution at various locations: at Ceara Rise in the
western equatorial Atlantic (Martin and Sayles, 1996), in the Arabian Sea (Milliman et al.,
1999), in the equatorial Indian Ocean (Peterson and Prell, 1985; Schulte and Bard, 2003), and
in the equatorial Pacific (Kimoto et al., 2003). Despite the evidence for supralysoclinal
dissolution in some areas of the world´s ocean, the question still exists whether dissolution
occurs above the lysocline in the entire ocean. The first part of this thesis seeks answers to
this question, based on the global budget model of Milliman et al. (1999). As study area the
Bahamas and Florida Straits are most suitable because of the high production of carbonate,
and because there the depth of the lysocline is the deepest worldwide. To monitor the
occurrence of supralysoclinal dissolution, the preservation of aragonitic pteropod shells was
determined, using the Limacina inflata Dissolution Index (LDX; Gerhardt and Henrich,
VAbstract
2001). Analyses of the grain-size distribution, the mineralogy, and the foraminifera
assemblage revealed further aspects concerning the preservation state of the sediment. All
samples located at the Bahamian platform are well preserved. In contrast, the samples from
the Florida Straits show dissolution in 800 to 1000 m and below 1500 m water depth.
Degradation of organic material and the subsequent release of CO probably causes 2
supralysoclinal dissolution. A northward extension of the corrosive Antarctic Intermediate
Water (AAIW) flows through the Caribbean Sea into the Gulf of Mexico and might enhance
dissolution processes at around 1000 m water depth.
The second part of this study deals with the preservation of Pliocene to Holocene
carbonate sediments from both the windward and leeward basins adjacent to Great Bahama
Bank (Ocean Drilling Program Sites 632, 633, and 1006). Detailed census counts of the sand
fraction (250-500 µm) show the general composition of the coarse grained sediment. Further
methods used to examine the preservation state of carbonates include the amount of organic
carbon and various dissolution indices, such as the LDX and the Fragmentation Index.
Carbonate concretions (nodules) have been observed in the sand fraction. They are similar to
the concretions or aggregates previously mentioned by Mullins et al. (1980a) and Droxler et
al. (1988a), respectively. Nonetheless, a detailed study of such grains has not been made to
date, although they form an important part of periplatform sediments. Stable isotope-
measurements of the nodules´ matrix confirm previous suggestions that the nodules have
formed in situ as a result of early diagenetic processes (Mullins et al., 1980a). The two cores,
which are located in Exuma Sound (Sites 632 and 633), at the eastern margin of Great
Bahama Bank (GBB), show an increasing amount of nodules with increasing core depth. In
Pliocene sediments, the amount of nodules might rise up to 100%. In contrast, nodules only
occur within glacial stages in the deeper part of the studied core interval (between 30 and
70 mbsf) at Site 1006 on the western margin of GBB. Above this level the sediment is
constantly being flushed by bottom water, that might also contain corrosive AAIW, which
would hinder cementation. Fine carbonate particles (<63 µm) form the matrix of the nodules
and do therefore not contribute to the fine fraction. At the same time, the amount of the coarse
fraction (>63 µm) increases due to the nodule formation. The formation of nodules might
therefore significantly alter the grain-size distribution of the sediment. A direct comparison of
the amount of nodules with the grain-size distribution shows that core intervals with high
amounts of nodules are indeed coarser than the intervals with low amounts of nodules. On the
other hand, an initially coarser sediment might facilitate the formation of nodules, as a high
porosity and permeability enhances early diagenetic processes (Westphal et al., 1999). This
VIAbstract
suggestion was also confirmed: the glacial intervals at Site 1006 are interpreted to have
already been rather coarse prior to the formation of nodules. This assumption is based on the
grain-size distribution in the upper part of the core, which is not yet affected by diagenesis,
but also shows coarser sediment during the glacial stages. As expected, the coarser, glacial
deposits in the lower part of the core show the highest amounts of nodules. The same effect
was observed at Site 632, where turbidites cause distinct coarse layers and reveal higher
amounts of nodules than non-turbiditic sequences. Site 633 shows a different pattern: both the
amount of nodules and the coarseness of the sediment steadily increase with increasing core
depth.
Based on these sedimentological findings, the following model has been developed: a
grain-size pattern characterised by prominent coarse peaks (as observed at Sites 632 and
1006) is barely altered. The greatest coarsening effect due to the nodule formation will occur
in those layers, which have initially been coarser than the adjacent sediment intervals. In this
case, the overall trend of the grain-size pattern before and after formation of the nodules is
similar to each other. Although the sediment is altered due to diagenetic processes, grain size
could be used as a proxy for e.g. changes in the bottom-water current. The other case
described in the model is based on a consistent initial grain-size distribution, as observed at
Site 633. In this case, the nodule reflects the increasing diagenetic alteration with increasing
core depth rather than the initial grain-size pattern. In the latter scenario, the overall grain-size
trend is significantly changed which makes grain size unreliable as a proxy for any
palaeoenvironmental changes.
The results of this study contribute to the understanding of general sedimentation
processes in the periplatform realm: the preservation state of surface samples shows the
influence of supralysoclinal dissolution due to the degradation of organic matter and due to
the presence of corrosive water masses; the composition of the sand fraction shows the
alteration of the carbonate sediment due to early diagenetic processes. However, open
questions are how and when the alteration processes occur and how geochemical parameters,
such as the rise in alkalinity or the amount of strontium, are linked to them. These
geochemical parameters might reveal more information about the depth in the sediment
column, where dissolution and cementation processes occur.
VIIKurzfassung
Kurzfassung
Der Karbonatkreislauf im Ozean besteht im Wesentlichen aus der Produktion und der
Lösung bzw. Erhaltung karbonatischer Partikel in der Wassersäule und im Sediment. Ein
wichtiger Steuerfaktor für die Produkt