Stable isotopic composition and carbonate concentrations of sediments of the Cretaceous/Paleogene boundary in the Antarctic

Stable isotopic records across the Cretaceous/Paleogene (K/P) boundary in Maud Rise Holes 689B and 690C indicate that significant climatic changes occurred during the latest Cretaceous, beginning approximately 500 k.y. prior to the mass extinction event and the enrichment of iridium at the K/P boundary (66.4 Ma). An oxygen isotopic decrease of ~0.7 per mil - ~1.0 per mil is recorded in the Late Cretaceous planktonic and benthic foraminifers between 66.9 and 66.6 Ma. The negative isotope excursion was followed by a positive excursion of similar magnitude between 66.6 Ma (latest Cretaceous) and ~66.3 Ma (earliest Paleocene). No other isotopic excursions of this magnitude are recorded in the planktonic and benthic microfossil records 1.0 m.y prior to, and for 2.0 m.y following the mass extinction event at the K/P boundary. The magnitude and duration of these isotopic excursions were similar to those at the Paleocene/Eocene and Eocene/Oligocene boundaries. A major d13C excursion occurred 200 k.y. prior to the boundary, involving a positive shift in planktonic and benthic d13C of ~0.5 per mil - 0.75 per mil. Similar changes observed in other deep-sea sequences indicate that this reflected a global change in d13C of the oceanic total dissolved carbon (TDC) reservoir. The magnitude of this inferred carbon reservoir change and its association with high latitude surface-water temperature changes recorded in the d18O records implies that it was linked to global climate change through feedback loops in the carbon cycle. At the K/P boundary, the surface-to-deep water d13C gradient is reduced by approximately 0.6 per mil - ~0.2 per mil. However, unlike sequences elsewhere, the planktonic-benthic d13C gradient (Delta d13C) was not eliminated in the Antarctic. The surface-to-deep water gradient was re-established gradually during the 400 k.y. following the mass extinction. Full recovery of the Delta d13C occurred by ~60.0 Ma. In addition to the reduced vertical d13C gradient across the K/P boundary, there was a negative excursion in both planktonic and benthic d13C beginning approximately 100 k.y. after the boundary (66.3 Ma). This excursion resulted in benthic d13C values in the early Paleogene that were similar to those in the pre-K/P boundary intervals. This negative shift appears to reflect a change in the d13C of the oceanic TDC reservoir shift that may have resulted from reduced carbon burial and/or increased carbon flux to the oceans. Any model that attempts to explain the demise of the oceanic plankton at the end of the Cretaceous should consider the oceanic environmental changes that were occurring prior to the massive extinction event.

Carbonate preservation of sediment cores from the Great Bahama Bank

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 CO2; an anthropogenic rise of CO2 in the atmosphere would therefore also increase the amount of CO2 in the ocean. The increased amount of CO2 in the ocean, due to increasing CO2-emissions into the atmosphere since the industrial revolution, has been interpreted as "ocean acidification" (Caldeira and Wickett, 2003). Its alarming effects, such as dissolution and reduced CaCO3 formation, on reefs and other carbonate shell producing organisms form the topic of current discussions (Kolbert, 2006). Decreasing temperatures and increasing pressure and CO2 enhance the dissolution of 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, 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 CO2 probably causes 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 isotopemeasurements 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 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.

Strontium isotope analysis of seawater and at a mixture of planktic foraminifera during the Paleocene and Early Eocene from different holes of ODP Leg 2008

We refined the strontium isotope seawater curve for the Paleocene and early Eocene by analysis of samples recovered from the Walvis Ridge during Ocean Drilling Project (ODP) Leg 208. The highest 87Sr/86Sr values occurred in the earliest Paleocene at 65 Ma and generally decreased throughout the Paleocene, reaching minimum values between 53 and 51 Ma in the early Eocene before beginning to increase again at 50 Ma. A plausible explanation for the 87Sr/86Sr decrease between 65 and 51 Ma is increased rates of hydrothermal activity and/or the eruption and weathering of large igneous provinces (e.g., Deccan Traps and North Atlantic). Strontium isotope variations closely parallel sea level and benthic d18O changes during the late Paleocene and early Eocene, supporting previous studies linking tectonic reorganization and increased volcanism to high sea level, high CO2, and warm global temperatures.

(Table T1) Geochemistry of calcium carbonate veins from ODP Holes 206-1256C and 206-1256D

Drilling a complete deep crustal section has been a primary yet elusive goal since the inception of scientific ocean drilling. In situ ocean crustal sections would contribute enormously to our understanding of the formation and subsequent evolution of the ocean crust, in particular the interplay between magmatic, hydrothermal, and tectonic processes. Ocean Drilling Program (ODP) Leg 206 was the first of a multileg project to drill an in situ crustal section that penetrated the gabbroic rocks of the Cocos plate (6°44.2'N, 91°56.1'W), which formed ~15 m.y. ago on the East Pacific Rise during a period of superfast spreading (>200 mm/yr) (Wilson, Teagle, Acton, et al., 2003, doi:10.2973/odp.proc.ir.206.2003). During Leg 206, the upper 500 m of basement was cored in Holes 1256C and 1256D with moderate to high recovery rates. The igneous rocks recovered are predominantly thin (10 cm to 3 m) basalt flows separated by chilled margins. There are also several massive flows (>3 m thick), although their abundance decreases with depth in Hole 1256D, as well as minor pillow basalts, hyaloclastites, and rare dikes. The lavas have been slightly (<10%) altered by low-temperature hydrothermal fluids, which resulted in pervasive dark gray background alteration and precipitation of saponite, pyrite, silica, celadonite, and calcium carbonate veins. Here we present a geochemical analysis of the CaCO3 recovered from cores. The compositions of ridge flank fluids within superfast spreading crust will be determined from these data, following the approach of Hart et al. (1994, doi:10.1029/93JB02035), Yatabe et al. (2000, doi:10.2973/odp.proc.sr.168.003.2000), and Coggon et al. (2004, doi:10.1016/S0012-821X(03)00697-6).

Seawater carbonate chemistry and impacts of calcium and carbonate ion concentration on Mg and Sr incorporation in cultured benthic foraminifera (Ammonia tepida and Heterostegina depressa), 2011

Laboratory culture experiments were conducted to determine effects of seawater carbonate ion concentration ([CO32-]), and thereby calcite saturation state, on Mg and Sr incorporation into calcite of two species of shallow-water benthic foraminifera: Ammonia tepida and Heterostegina depressa. Impact on Mg and Sr incorporation by increased seawater [CO32-] and thereby higher calcite saturation state, is absent in either species. Comparison to results from a similar culturing experiment, in which calcite saturation state was varied as a function of [Ca2+], reveals that saturation state affects incorporation of Mg and Sr through calcium- rather than carbonate availability. The similarity in response by both species is surprising since the average Mg/Ca ratio is ~ 70 times higher in H. depressa than in A. tepida. Furthermore, these results suggest that the ions involved in biomineralization (i.e. Ca2+ and DIC) are processed by separate cellular transport mechanisms. The similar response of Mg and Sr incorporation in this study suggests that only differences in the Ca2+ transport mechanism affect divalent cation partitioning.

Chemical and isotopic composition of carbonate veins from ODP Leg 209

Carbonate veins hosted in ultramafic basement drilled at two sites in the Mid Atlantic Ridge 15°N area record two different stages of fluid-basement interaction. A first generation of carbonate veins consists of calcite and dolomite that formed syn- to postkinematically in tremolite-chlorite schists and serpentine schists that represent gently dipping large-offset faults. These veins formed at temperatures between 90 and 170 °C (oxygen isotope thermometry) and from fluids that show intense exchange of Sr and Li with the basement (87Sr/86Sr = 0.70387 to 0.70641, d7Li L-SVEC = + 3.3 to + 8.6 per mil). Carbon isotopic compositions range to high d13C PDB values (+ 8.7 per mil), indicating that methanogenesis took place at depth. The Sr-Li-C isotopic composition suggests temperatures of fluid-rock interaction that are much higher (T > 350-400 °C) than the temperatures of vein mineral precipitation inferred from oxygen isotopes. A possible explanation for this discrepancy is that fluids cooled conductively during upflow within the presumed detachment fault. Aragonite veins were formed during the last 130 kyrs at low-temperatures within the uplifted serpentinized peridotites. Chemical and isotopic data suggest that the aragonites precipitated from cold seawater, which underwent overall little exchange with the basement. Oxygen isotope compositions indicate an increase in formation temperature of the veins by 8-12 °C within the uppermost ~ 80 m of the subseafloor. This increase corresponds to a high regional geothermal gradient of 100-150 °C/km, characteristic of young lithosphere undergoing rapid uplift.