Chemistry of Jurassic/Cretaceous black shales

Black shale samples of Jurassic to Cretaceous age recovered during the 'Norwegian Shelf Drilling Program' between 1987 and 1991 from Sites 7430/10-U-01 (Barents Sea), 6814/04-U-02 (Norwegian Shelf near the Lofoten) and 6307/07-U-02 (Norwegian Shelf near Trondheim) were analyzed for major and trace elements. These laminated black shales are characterized by high total organic carbon (TOC) and total sulfur (TS) contents as well as by significant enrichments in several redox-sensitive and/or sulfide-forming trace metals (Ag, Bi, Cd, Co, Cr, Cu, Mo, Ni, Re, Sb, Tl, U, V, and Zn). Enrichment factors relative to 'average shale' are comparable to those found in Cenomanian-Turonian boundary event (CTBE) black shales and Mediterranean sapropels. The Re content is high in the studied black shales, with maximum values up to 1221 ng/g. Re/Mo ratios averaging 2.3*10**-3 are close to the seawater value. High trace metal enrichments and Re/Mo ratios close to the seawater value point to a dominantly anoxic and sulfidic water column during black shale formation. Interbeds with higher Re/Mo ratios, especially in high-resolution sampled core sections, point to brief periods of suboxic conditions. Additionally, enhanced Zn concentrations in the black shales from the Barents Sea support the assumption that hydrothermal activity was also high during black shale deposition. Trace metal signatures of black shales at different drill sites on a transect along the Norwegian Shelf are not only influenced by water depth but also by their location in the boreal realm. Metal enrichments are higher in the northern compared to the southern sites. Volgian (=Tithonian 151-144 Ma BP) black shales exhibit elevated trace metal contents in comparison to their Berriasian (144-137 Ma BP) counterparts. This probably reflects a change in the circulation pattern during periods of black shale formation. Therefore different paleoceanographic conditions, probably controlled by climatic change linked to the transgression of the paleo-sealevel and the North Atlantic opening, may have developed from the Volgian to the Berriasian.

Lacustrine sediments from two setions in northern Greece

We investigated two lignite quarries in northern Greece for orbital and suborbital climate variability. Sections Lava and Vegora are located at the southern and northern boundaries of the Ptolemais Basin, a northwest southeast elongated intramontane basin that contains Upper Miocene to Lower Pliocene lacustrine sediments. Sediments show cyclic alterations of marl-rich (light), and coal-rich or clay-rich (dark) strata on a decimeter to meter scale. First, we established low-resolution ground-truth stratigraphy based on paleomagnetics and biostratigraphy. Accordingly, the lower 67 m and 65 m that were investigated in both sections Vegora and Lava, respectively, belong to the Upper Miocene and cover a time period of 6.85 to 6.57 and 6.46 to 5.98 Ma at sedimentation rates of roughly 14 and 22 cm/ka. In order to obtain a robust and high-resolution chronology, we then tuned carbonate minima (low L* values; high magnetic susceptibility values) to insolation minima. Besides the known dominance of orbital precession and eccentricity, we detected a robust hemi-precessional cycle in most parameters, most likely indicative for monsoonal influence on climate. Moreover, the insolation-forced time series indicate a number of millennial-scale frequencies that are statistically significant with dominant periods of 1.5-8 kyr. Evolutionary spectral analysis indicates that millennial-scale climate variability documented for the Ptolemais Basin resembles the one that is preserved in ice-core records of Greenland. Most cycles show durations of several tens of thousands of years before they diminish or cease. This is surprising because the generally argued cause for Late Quaternary millennial-scale variability is associated with the presence of large ice sheets, which cannot be the case for the Upper Miocene. Possible explanations maybe a direct response to solar forcing, an influence on the formation of North Atlantic Deep Water through the outflow of high-salinity water, or an atmospheric link to the North Atlantic Oscillation.

Age determination and grain size distribution of sediment core GeoB3375-1

The palaeoclimatic conditions during the Last Glacial Maximum (LGM) of southern South America and especially latitudinal shifts of the southern westerly wind belt are still discussed controversially. Longer palaeoclimatic records covering the Late Quaternary are rare. A particularly sensitive area to Late Quaternary climatic changes is the Norte Chico, northern Chile, because of its extreme climatic gradients. Small shifts of the present climatic zonation could cause significant variations of the terrestrial sedimentary environment which would be recorded in marine terrigenous sediments. To unveil the history of shifting climatic zones in northern Chile, we present a sedimentological study of a marine sediment core (GeoB 3375-1) from the continental slope off the Norte Chico (27.5°S). Sedimentological investigations include bulk- and silt grain-size determinations by sieving, Atterberg separation, and detailed SediGraph analyses. Additionally, clay mineralogical parameters were obtained by X-ray diffraction methods. The 14C-dated core, covering the time span from approximately 10,000 to 120,000 cal. yr B.P., consists of hemipelagic sediments. Terrigenous sedimentological parameters reveal a strong cyclicity, which is interpreted in terms of variations of sediment provenance, modifications of the terrestrial weathering regimes, and modes of sediment input to the ocean. These interpretations imply cyclic variations between comparatively arid climates and more humid conditions with seasonal precipitation for northern Chile (27.5°S) through the Late Quaternary. The cyclicity of the terrigenous sediment parameters is strongly dominated by precessional cycles. For the palaeoclimatic signal, this means that more humid conditions coincide with maxima of the precession index, as e.g. during the LGM. Higher seasonal precipitation for this part of Chile is most likely derived from frontal winter rain of the Southern Westerlies. Thus, the data presented here favour not only an equatorward shift of this atmospheric circulation system during the LGM, but also precession-controlled latitudinal movements throughout the Late Quaternary. Precessional forcing of latitudinal movements of the westerly atmospheric circulation system may be conceivable through teleconnections to the Northern Hemisphere monsoonal system in the Atlantic Ocean region.

Paleoceanographic record since 13kyr BP of sediment core LaPAS-KF02

In this paper we present a paleoceanographic reconstruction of the southwestern South Atlantic for the past 13 kyr based on faunal and isotopic analysis of planktonic foraminifera from a high-resolution core retrieved at the South Brazil Bight continental slope. Our record indicates that oceanographic changes in the southwestern South Atlantic during the onset of the Holocene were comparable in strength to those that occurred during the Younger Dryas. Full interglacial conditions started abruptly after 8.2 kyr BP with a sharp change in faunal composition and surface hydrography (SST and SSS). Part of the observed events may be explained in terms of changes in thermohaline circulation while the other part suggests a dominant role of winds. Our data indicate that during the Early Holocene upwelling was significantly strengthened in the South Brazil Bight promoting high productivity and preventing the establishment of the typically interglacial menardiiform species. In general terms, oceanographic changes recorded by core KF02 occurred in synchrony with Antarctica's climate.

Siliceous microfossils in middle Eocene sediments of ODP Holes 120-748B and 120-749B

The Middle Eocene Climatic Optimum (MECO) is a major transient warming event that occurred at ~ 40 Ma and reversed a long-term cooling trend through the early and middle Eocene. We report the results of a high-resolution, quantitative study of siliceous microfossils at Ocean Drilling Program Sites 748 and 749 (Southern Kerguelen Plateau, Southern Ocean, ~ 58°S) across a ~ 1.4 myr interval spanning the MECO event. At both sites, a significant increase in biosiliceous sedimentation is associated with the MECO event. Rich siliceous planktonic microfossil assemblages in this interval are unusual in that they are dominated by ebridians, with radiolarians as a secondary major component. Silicoflagellates and diatoms comprise only a minor fraction of the assemblage, in contrast to siliceous microfossil assemblages that characterize modern Southern Ocean sediments. Based on our new siliceous microfossil records, we interpret two ~ 300 kyr periods of elevated nutrient availability in Southern Ocean surface waters which span the peak warming interval of the MECO and the post-MECO cooling interval. A diverse assemblage of large silicoflagellates belonging to the Dictyocha grandis plexus is linked to the rapid rise in sea-surface temperatures immediately prior to peak warmth, and a pronounced turnover is observed in both ebridian and silicoflagellate assemblages at the onset of peak warming. The interval of peak warmth is also characterized by high abundance of cosmopolitan ebridians (e.g., Ammodochium spp.) and silicoflagellates (e.g., Naviculopsis spp.), and increased abundance of tropical and subtropical diatom genera (e.g., Asterolampra and Azpeitia). These observations confirm the relative pattern of temperature change interpreted from geochemical proxy data at multiple Southern Ocean sites. Furthermore, rapid assemblage changes in both autotrophic and heterotrophic siliceous microfossil groups indicate a reorganization of Southern Ocean plankton communities in response to greenhouse warming during the MECO event.

Plio-Pleistocene age-depth points and occurrence of hiatuses in ODP sites of the Southern Ocean

Changes in Atlantic deep water circulation were reconstructed by comparing the benthic foraminiferal delta13C record at ODP Site 1090 in the South Atlantic with similar records from the North Atlantic (Sites 982, 607, 925, 929) and deep Pacific (Site 849) oceans. Important deep water circulation changes occurred in the early Pleistocene at 1.55 Myr and during the Mid-Pleistocene Transition at 0.9 Myr. At 1.55 Myr, glacial delta13C values in the Southern Ocean became significantly lower than those in the deep Pacific, establishing a pattern that persisted throughout the late Pleistocene. We propose that the lowering of delta13C values of Southern Component Water (SCW) at this time resulted from expansion of sea ice and reduced ventilation of deep water during glacial periods after marine isotope stage 52. Accompanying this change in Southern Ocean deep water circulation was enhanced interhemispheric coupling between the North and South Atlantic after 1.55 Myr. At ~0.9 Myr, the magnitude of glacial-to-interglacial variabilityin delta13C increased and shifted to a longer frequency (100 kyr) along with oceanic delta18O (ice volume). Calculation of percent Northern Component Water (NCW) using Site 1090 as the SCW end member yielded 20-30% less reduction of NCW during glacial periods of the late Pleistocene. Also, a trend toward reduced glacial suppression of NCW during the past 400 kyr is not evident. The apparent decoupling of ice volume and deep water circulation reported previously maybe an artifact of using a Pacific, rather than a Southern Ocean, carbon isotopic record to calculate past mixing ratios of NCW and SCW.

(Table 1) Stable isotope data of chemoherm carbonates at Hydrate Ridge on the Cascadia continental margin

Two active chemoherm build-ups growing freely up into the oceanic water column, the Pinnacle and the South East-Knoll Chemoherms, have been discovered at Hydrate Ridge on the Cascadia continental margin. These microbially-mediated carbonate formations rise above the seafloor by several tens of meters and display a pinnacle-shaped morphology with steep flanks. The recovered rocks are pure carbonates dominated by aragonite. Based on fabric and mineralogic composition different varieties of authigenic aragonite can be distinguished. Detailed visual and petrographic investigations unambiguously reveal the involvement of microbes during the formation of the carbonates. The fabric of the cryptocrystalline and fibrous aragonite can be described as thrombolitic. Fossilized microbial filaments in the microcrystalline aragonite indicate the intimate relationship between microbes and carbonates. The strongly 13C-depleted carbon isotope values of the samples (as low as -48.1 per mill PDB) are characteristic of methane as the major carbon source for the carbonate formation. The methane-rich fluids from which the carbonates are precipitated originate most probably from a gas reservoir below the bottom-simulating reflector (BSR) and rise through fault systems. The d18O values of the aragonitic chemoherm carbonates are substantially higher (as high as 5.0 per mill PDB) than the expected equilibrium value for an aragonite forming from ambient seawater (3.5 per mill PDB). As a first approximation this indicates formation from glacial ocean water but other factors are considered as well. A conceptual model is presented for the precipitation of these chemoherm carbonates based on in situ observations and the detailed petrographic investigation of the carbonates. This model explains the function of the consortium of archaea and sulfate-reducing bacteria that grows on the carbonates performing anaerobic oxidation of methane (AOM) and enabling the precipitation of the chemoherms above the seafloor surrounded by oxic seawater. Beggiatoa mats growing on the surface of the chemoherms oxidize the sulfide provided by sulfate-dependent anaerobic oxidation of methane within an oxic environment. The contact between Beggiatoa and the underlying microbial consortium represents the interface between the overlying oxic water column and an anoxic micro-environment where carbonate formation takes place.

Late Miocene-early Pliocene stable isotope record of benthic foraminifera from ODP Site 177-1088 in the sub-Antarctic South Atlantic

Benthic foraminiferal stable isotope records for the past 11 Myr from a recently drilled site in the sub-Antarctic South Atlantic (Site 1088, Ocean Drilling Program Leg 177, 41°S, 15°E, 2082 m water depth) provide, for the first time, a continuous long-term perspective on deep water distribution patterns and Southern Ocean climate change from the late Miocene through the early Pliocene. I have compiled published late Miocene through Pliocene stable isotope records to place the new South Atlantic record in a global framework. Carbon isotope gradients between the North Atlantic, South Atlantic, and Pacific indicate that a nutrient-depleted watermass, probably of North Atlantic origin, reached the sub-Antarctic South Atlantic after 6.6 Ma. By 6.0 Ma the relative proportion of the northern-provenance watermass was similar to today and by the early Pliocene it had increased to greater than the modern proportion suggesting that thermohaline overturn in the Atlantic was relatively strong prior to the early Pliocene interval of inferred climatic warmth. Site 1088 oxygen isotope values display a two-step increase between ~7.4 Ma and 6.9 Ma, a trend that parallels a published delta18O record of a site on the Atlantic coast of Morocco. This is perhaps best explained by a gradual cooling of watermasses that were sinking in the Southern Ocean. I speculate that relatively strong thermohaline overturn at rates comparable to the present day interglacial interval during the latest Miocene may have provided the initial conditions for early Pliocene climatic warmth. The impact of an emerging Central American Seaway on Atlantic-Pacific Ocean upper water exchange may have been felt in the North Atlantic beginning in the latest Miocene between 6.6 and 6.0 Ma, which would be ~1.5 Myr earlier than previously thought.