Hans-Jürgen Brumsack

Stratigraphy, Geochemistry, and Paleoceanography of Organic Carbon-Rich Cretaceous Sequences

The Cretaceous is characterized by unusually widespread distribution of “black shales”-- sequences of variable lithology containing numerous beds with organic-carbon (OC) contents in excess of 1 percent by weight-- in both deep- and shallow-marine settings. General time envelopes of globally important organic-carbon burial during the Aptian-Albian, at the Cenomanian-Turonian boundary, and to a lesser extent in the Coniacian-Santonian, have been termed “Oceanic Anoxic Events (OAEs)”. The available stratigraphic and organic geochemical data and secular trends in the carbon isotopic composition of marine carbonates suggest that the timing of OC burial is broadly synchronous during these episodes and that the mass of OC buried during each is considerable. There may be other important episodes of more widespread OC burial, as yet poorly documented, such as the Valanginian- Hauterivian.

Barite fronts in continental margin sediments: a new look at barium remobilization in the zone of sulfate reduction and formation of heavy barites in diagenetic fronts

Micro-crystalline barites recovered by deep-sea drilling from Site 684 on the Peru margin and Site 799 in the Japan Sea are highly enriched in the heavy sulfur isotope relative to seawater (δ34S up to + 84%). This isotopic composition is consistent with remobilization of biogenic barite triggered by sulfate reduction, and subsequent reprecipitation as a diagenetic barite front. The high levels of barium sulfate in these deposits (10–50%) cannot be explained by a diffusive transport model in sediments experiencing a constant rate of sedimentation. When sedimentation rates change radically, the barite front will remain at a given depth interval leading to large accumulations of barium sulfate. Such conditions may have generated the barite deposits at Site 799. At Site 684, on the other hand, there is evidence that the barite deposits are a result of the tectonically-driven advection of sulfate-bearing fluids through the sediment column.

Black shale deposition on the northwest African Shelf during the Cenomanian/Turonian oceanic anoxic event: Climate coupling and global organic carbon burial

High-resolution geochemical records from a depth transect through the Cenomanian/Turonian (C/T) Tarfaya Basin (northwest African Shelf) reveal high-amplitude fluctuations in accumulation rates of organic carbon (OC), redox-sensitive and sulphide-forming trace metals, and biomarkers indicative of photic zone euxinia. These fluctuations are in general coeval and thus imply a strong relationship of OC burial and water column redox conditions. The pacing and regularity of the records and the absence of a prominent continental signature suggest a dynamic depositional setting linked to orbital and higher-frequency forcing. Determining the dominant frequency depends on the definition of the most pronounced oceanic anoxic event (OAE2) and its duration. We propose that eccentricity is the main forcing factor at Tarfaya and controlled fluctuations in wind-driven upwelling of nutrient-rich, oxygen-depleted intermediate waters from the adjacent Atlantic and the periodic development of photic zone and bottom water euxinia on the mid-Cretaceous northwest African shelf. Accumulation records clearly identify the basin center as the primary site of sediment deposition with highest temporal variability and an up to six-fold increase in OC burial from similar to2 g/m(2) . yr prior to the OAE2 to similar to12 g/m(2) . yr during the OAE2. Photic zone and bottom water euxinia alternated with periods of greater oxygenation of the water column in response to climate forcing. Mass balance calculations imply that similar to2% of the overall global excess OC burial associated with the OAE2 was deposited in the Tarfaya Basin, an area that represented only similar to0.05% of the total global C/T ocean floor. In fact, the lateral extent of similar black shales along the African continental margin indicates that this part of the ocean contributed significantly to the global increase in organic carbon burial during the OAE2.

The sapropel record of the eastern Mediterranean Sea — results of Ocean Drilling Program Leg 160

Research on sediments recovered during Ocean Drilling Leg 160 has concentrated on two issues: the first concerned the stratigraphy of sapropel formation, the second was oriented to clarify specific processes that explain sapropel origin. Progress has been made in the construction of stratigraphic composites out of sedimentary sequences from individual holes at each of the palaeoceanographic sites. On the composites, initial work has resulted in the establishment of high-resolution and intermediate-resolution stratigraphies for three sites (963, 964, 967); correlation of sedimentary cycles to astronomical (insolation) cycles extends the stratigraphies to Sites 969 and 966. The sapropel occurrences in the marine and land sequences over the entire Eastern Mediterranean are correlated; with the resolution that can be obtained from isotope studies, groups of sapropels occurred simultaneously over the entire basin. In detail, however, the temporal and facies patterns of sapropel sequences differ between individual sites and depositional basins. The differences may be related to effects of water depth, diagenesis, and post-depositional tectonic attenuation of sequences. Studies on the geochemistry and facies of sapropels agree that anoxic conditions favoured preservation of organic matter in sapropels, caused the enrichment of trace metals associated with sapropels, and helped to preserve primary sedimentary structures. Besides, all evidence is consistent with elevated fluxes of organic matter and associated elements during sapropel events.

Geochemistry of Cretaceous black shales from the Atlantic Pcean (DSDP Legs 11, 14, 36 and 41)☆

Eighty-eight samples of Cretaceous sediments from the Atlantic Ocean (DSDP Sites 105, 135, 137, 138, 144, 330, 367, 368 and 370), including 38 black shale samples with organic C contents of more than 1% were analysed mostly by spectrometric methods (relative standard deviation 2–15%) for the elements Fe, Mn, S, Zn, Cu, Cr, Pb, Ni, Co, Mo, V, Ba and Sr, and in selected samples for Ag, Cd, Tl, Bi and B. The high accumulation of heavy metals relative to average shale (factors up to 330) cannot be explained by the following processes: (1) water composition of marginal basins controlled by continental runoff, (2) hydrothermal solutions connected with ridge volcanism, or (3) metabolism of marine organisms. The order of heavy-metal enrichment relative to average shale (Pb < Co < Cr < Ni < Cu < V < Zn < Mo < Ag) corresponds well (except Ag) with the increasing concentration of these elements in normal seawater. The presence of pyrite, sphalerite and Cu-sulfides could be demonstrated by microprobe scanning. The bacteriogenic origin of sulfur could be recognized by δ 34S analysis. Pyrite and the other heavy-metal sulfides were not in isotopic equilibrium. This is an indication of the heavy-metal sulfides being formed in the water column and pyrite in the sediments. The Cretaceous Proto-Atlantic can be compared to the present euxynic stage of the Black Sea. Model calculations show that a normal productivity is sufficient for the explanation of the relatively high organic C content (up to 35%). During relatively short periods of stagnation (1–10 Ma) considerable amounts of organic carbon (7 · 1013 t), sulfur (2 · 1013 t) and heavy metals (9.1 · 1011−5 · 109 t) had been fixed in the black shales.

Arctic late Paleocene-early Eocene paleoenvironments with special emphasis on the Paleocene-Eocene thermal maximum (Lomonosov Ridge, Integrated Ocean Drilling Program Expedition 302)

We reconstruct the latest Paleocene and early Eocene (∼57-50 Ma) environmental trends in the Arctic Ocean and focus on the Paleocene-Eocene thermal maximum (PETM) (∼55 Ma), using strata recovered from the Lomonosov Ridge by the Integrated Ocean Drilling Program Expedition 302. The Lomonosov Ridge was still partially subaerial during the latest Paleocene and earliest Eocene and gradually subsided during the early Eocene. Organic dinoflagellate cyst (dinocyst) assemblages point to brackish and productive surface waters throughout the latest Paleocene and early Eocene. Dinocyst assemblages are cosmopolitan during this time interval, suggesting warm conditions, which is corroborated by TEX86′-reconstructed temperatures of 15°-18°C. Inorganic geochemistry generally reflects reducing conditions within the sediment and euxinic conditions during the upper lower Eocene. Spectral analysis reveals that the cyclicity, recorded in X-ray fluorescence scanning Fe data from close to Eocene thermal maximum 2 (∼53 Ma, presence confirmed by dinocyst stratigraphy), is related to precession. Within the lower part of the PETM, proxy records indicate enhanced weathering, runoff, anoxia, and productivity along with sea level rise. On the basis of total organic carbon content and variations in sediment accumulation rates, excess organic carbon burial in the Arctic Ocean appears to have contributed significantly to the sequestration of injected carbon during the PETM.

Mid-Cenozoic tectonic and paleoenvironmental setting of the central Arctic Ocean

Drilling results from the Integrated Ocean Drilling Program’s Arctic Coring Expedition (ACEX) to the Lomonosov Ridge (LR) document a 26 million year hiatus that separates freshwater-influenced biosilica-rich deposits of the middle Eocene from fossil-poor glaciomarine silty clays of the early Miocene. Detailed micropaleontological and sedimentological data from sediments surrounding this mid-Cenozoic hiatus describe a shallow water setting for the LR, a finding that conflicts with predrilling seismic predictions and an initial postcruise assessment of its subsidence history that assumed smooth thermally controlled subsidence following rifting. A review of Cenozoic tectonic processes affecting the geodynamic evolution of the central Arctic Ocean highlights a prolonged phase of basin-wide compression that ended in the early Miocene. The coincidence in timing between the end of compression and the start of rapid early Miocene subsidence provides a compelling link between these observations and similarly accounts for the shallow water setting that persisted more than 30 million years after rifting ended. However, for much of the late Paleogene and early Neogene, tectonic reconstructions of the Arctic Ocean describe a landlocked basin, adding additional uncertainty to reconstructions of paleodepth estimates as the magnitude of regional sea level variations remains unknown.

Astronomical forcing of the East Asian monsoon mirrored by the composition of Pliocene South China Sea sediments

Abstract High-precision major and minor element records of Pliocene core intervals from Ocean Drilling Program (ODP) Site 1145 located in the northern part of the South China Sea were generated at a 5 cm (approximately 2 kyr) resolution. From the geochemical data distinct proxies, those indicating changes in detrital matter provenance as well as those indicating productivity variations, could be derived. Both the terrigenous input to the northern South China Sea (K/Si) and variations in productivity (Ba/Al) show a response to insolation-forced monsoon variability. While the fluvial input (K/Si) responds to changes in the summer monsoon, productivity increases, as documented by Ba enrichments, seem to reflect variations in winter monsoon intensity. A stronger winter monsoon may have increased nutrient availability via dust input and/or upwelling phenomena. According to our geochemical proxy records, summer and winter monsoons are approximately 180° out-of-phase. This may imply forcing of the winter monsoon through Southern Hemisphere summer insolation maxima. The K/Si ratio shows a linear correlation with the La90(1,0.5) Northern Hemisphere summer insolation record and has therefore been used to create an astronomical timescale for the investigated sediments of ODP Site 1145. Carbonate contents and corresponding color reflectances display 41 and 100 kyr cycles. We propose that these do not reflect glacial cycles, but are rather the result of an interference signal of dilution by fluvial and eolian terrigenous material and carbonate production (burial flux). A sudden onset of gradual changes in sediment composition occurs after 3.0 Ma. This is also documented by an increase in sedimentation rates and may be caused by enhanced fluvial input from the Pearl River due to intensified weathering rates on the Asian continent. This may be linked to tectonic processes around the Himalayan–Tibetan Complex and its consequences regarding land–sea heating and atmospheric circulation.

Organic matter and trace element-rich sapropels and black shales: A geochemical comparison

A distinct Pliocene eastern Mediterranean sapropel (i-282), recovered from three Ocean Drilling Program (ODP) Leg 160 Sites, has been investigated for its organic and inorganic composition. This sapropel is characterized by high organic carbon (Corg) and trace element contents, and the presence of isorenieratene derivatives. The latter suggests that the base of the photic zone was sulphidic during formation of the sapropel. Combined with evidence of bottom water anoxia (preservation of laminae, high redox-sensitive trace element contents, and the abundance and isotopic composition of pyrite) this leads to the tentative conclusion that almost the entire water column may have been anoxic. This anoxia resulted from high productivity and not from stagnation, because an approximation of the trace element budget during sapropel formation shows that water exchange with the western Mediterranean is needed. Entire water column anoxia has been suggested earlier for several black shales. With regard to the depositional environment and the Corg content, however, only the Cenomanian/Turonian Boundary Event (CTBE) black shales appear to be comparable to this sapropel. The proposed trace element removal mechanism of scavenging and (co-)precipitation in an anoxic water column, is thought to be similar for both types of deposits. The ultimate trace element source for the sapropel, however, is seawater, whereas it is hydrothermal and fluvial input for CTBE black shales (because they have a larger temporal and spatial distribution). Nonetheless, the Corg-rich eastern Mediterranean Pliocene sapropel discussed here may be considered to be a younger analogue of CTBE black shales.

Geochemical characteristics of deep-sea sediments from the Arabian Sea: a high-resolution study

Abstract Five deep-sea cores from the Arabian Sea, covering a time interval of 170 ky, were sampled at high resolution and analysed for major and trace elements. The stratigraphy of the cores was obtained by comparing the Ba/Al ratio with the SPECMAP data. This method is based on the close correlation between Ba concentrations (or Ba/Al ratios) and oxygen isotope ratios (δ18O) of foraminifera in Arabian Sea sediments. Ba/Al ratios, used as a productivity proxy, are variable but high in all deep-sea cores of the Arabian Sea, indicating a basin-wide influence of nutrient-rich water masses. Compared to glacial intervals the warmer periods are characterised by higher element/Al ratios of proxies, which are directly (Ba, Ca, Sr, P, i.e. hard parts of organisms) or indirectly (U, 230Thex) related to biological productivity. 230Thex provides evidence for intense boundary scavenging caused by high productivity due to enhanced upwelling and terrigenous input from the Indus fan. The Mn distribution in a core from the western Arabian Sea shows enrichments during interglacial periods and may indicate Mn export owing to the presence of a stronger oxygen-minimum zone. Several processes limit the applicability of proxies. Intercalated turbidites in the deep-sea cores can be identified by means of Si/Al, Ti/Al and Zr/Al ratios versus depth due to the enrichment of quartz, Ti-minerals and zircon in the basal layer. These elements are also important for the reconstruction of the history and extent of the eolian dust input. Diagenetic redistribution of redox sensitive elements occurs at the boundaries between turbidites and “normal” pelagic sedimentation and may mask or destroy primary signals.