Marcus Lipinski

Trace metals in Holocene coastal peats and their relation to pyrite formation (NW Germany)

Abstract Three drill cores from the marshlands of NW Germany, which cover the entire Holocene, were analyzed at high-resolution for bulk composition, Al, Fe, selected trace metals, and stable sulfur isotopes. The drill cores contain two lithological types of peat: (i) basal peats overlying Pleistocene sands and (ii) intercalated peats situated between clastic sediments of predominantly marine origin. The peat layers are characterized by distinct enrichments in pyrite due to microbial sulfate reduction under almost open system conditions with respect to seawater sulfate as shown by sulfur isotope partitioning. The main Fe source seems to be the freshwater environment. The determination of dissolved and particulate Fe of channels and small rivers close to the study area revealed a 50-fold higher Fe content of the freshwater environment when compared with North Sea water. Pyrite enrichments are explained by two scenarios: (i) pyrite formation coincides with fen reed peat growth (basal and intercalated) under the influence of a brackish water zone (salinity app. 5–15) and (ii) pyrite was formed after peat growth in the lowest limnic basal peat intervals. Maximum pyrite accumulation (TS 28%) occurs in latter peats that contain thin clastic layers as a result of tidal channel activities after peat formation. The occurrence of clastic layers may have favoured the inflow of saline groundwater. The peat layers are also characterized by enrichments in redox-sensitive trace metals (As, Mo, Re, U) and Cd, whereas Co, Cr, Cu, Mn, Ni, Pb, Tl, and Zn reflect the geogenic background. Leaching experiments have shown that As, Co, Cu, Mo, Re, and Tl are predominantly fixed as sulfides and/or incorporated into pyrite. The remaining trace metals show no distinct trends, only Cr reveals a strong relation to the lithogenic detritus. Seawater is the dominating source for As, Cd, Mo, Re, and U. The remaining trace elements seem to have a freshwater source similar to Fe. In contrast to the distribution of pyrite, highest amounts of redox-sensitive trace metals are seen in fen reed peats (basal and intercalated) that were formed under a direct influence of seawater and brackish water, respectively. Therefore, we suggest that saline groundwater entering the basal peats was probably depleted in redox-sensitive trace metals, e.g. owing to microbially induced reduction of trace metals and subsequent precipitation as sulfides or fixation by organic matter.

The Greenland-Norwegian Seaway: a key area for understanding Late Jurassic to early Cretaceous paleoenvironments

The paleoclimatology and paleoceanology of the Late Jurassic and Early Cretaceous are of special interest because this was a time when large amounts of marine organic matter were deposited in sediments that have subsequently become petroleum source rocks. However, because of the lack of outcrops, most studies have concentrated on low latitudes, in particular the Tethys and the “Boreal Realm,” where information has been based largely on material from northwest Germany, the North Sea, and England. These areas were all south of 40°N latitude during the Late Jurassic and Early Cretaceous. We have studied sediment samples of Kimmeridgian (∼154 Ma) to Barremian (∼121 Ma) age from cores taken at sites offshore mid-Norway and in the Barents Sea that lay in a narrow seaway connecting the Tethys with the northern polar ocean. During the Late Jurassic-Early Cretaceous these sites had paleolatitudes of 42–67°N. The Late Jurassic-Early Cretaceous sequences at these sites reflect the global sea-level rise during the Volgian-Hauterivian and a climatic shift from warm humid conditions in Volgian times to arid cold climates in the early Hauterivian. The sediments indicate orbital control of climate, reflected in fluctuations in the clastic influx and variations in carbonate and organic matter production. Trace element concentrations in the Volgian-Berriasian sediments suggest that the central part of the Greenland-Norwegian Seaway might have had suboxic bottom water beneath an oxic water column. Both marine and terrigenous organic matter are present in the seaway sediments. The Volgian-Berriasian strata have unusually high contents of organic carbon and are the source rocks for petroleum and gas fields in the region. The accumulation of organic carbon is attributed to restricted conditions in the seaway during this time of low sea level. It might be that the Greenland-Norwegian segment was the deepest part of the transcontinental seaway, bounded at both ends by relatively shallow swells. The decline in organic matter content of the sediments in the Valanginian-Hauterivian indicates greater ventilation and more active flow through the seaway as the sea level rose. The same benthic foraminifera assemblages are encountered throughout the seaway. Endemic assemblages of arenaceous foraminifera in the Volgian-Berriasian give way to more diverse and cosmopolitan Valanginian-Hauterivian benthic communities that include calcareous species. The foraminiferal assemblages also suggest low oxygen content bottom waters during the earlier Cretaceous, changing to more fully oxygenated conditions later. The calcareous nannoplankton, particularly Crucibiscutum salebrosum, which is rare at low latitudes and abundant in high latitudes, reflect the meridional thermal gradient. They indicate that the Greenland-Norwegian segment of the seaway was north of a subtropical frontal zone that acted as a barrier between the Tethyan and Boreal Realms. This implies the existence of stable climatic belts during the early Valanginian and Hauterivian, significant meridional temperature gradients, and moderate “ice house” conditions.

Trace metal signatures of Jurassic/Cretaceous black shales from the Norwegian Shelf and the Barents Sea

Abstract 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.

Late Jurassic to Early Cretaceous black shale formation and paleoenvironment in high northern latitudes: Examples from the Norwegian‐Greenland Seaway

[1] The Late Jurassic to Early Cretaceous (Volgian-Ryazanian) was a period of a second-order sea-level low stand, and it provided excellent conditions for the formation of shallow marine black shales in the NorwegianGreenland Seaway (NGS). IKU Petroleum Research drilling cores taken offshore along the Norwegian shelf were investigated with geochemical and microscopic approaches to (1) determine the composition of the organic matter, (2) characterize the depositional environments, and (3) discuss the mechanisms which may have controlled production, accumulation, and preservation of the organic matter. The black shale sequences show a wide range of organic carbon contents (0.5–7.0 wt %) and consist of thermally immature organic matter of type II to II/III kerogen. Rock-Eval pyrolysis revealed fair to very good petroleum source rock potential, suggesting a deposition in restricted shallow marine basins. Well-developed lamination and the formation of autochthonous pyrite framboids further indicate suboxic to anoxic bottom water conditions. In combination with very low sedimentation rates it seems likely that preservation was the principal control on organic matter accumulation. However, a decrease of organic carbon preservation and an increase of refractory organic matter from the Volgian to the Hauterivian are superimposed on short-term variations (probably reflecting Milankovitch cycles). Various parameters indicate that black shale formation in the NGS was gradually terminated by increased oxidative conditions in the course of a sea-level rise. INDEX TERMS: 1055 Geochemistry: Organic geochemistry; 4802 Oceanography: Biological and Chemical: Anoxic environments; 9609 Information Related to Geologic Time: Mesozoic; 8105 Tectonophysics: Continental margins and sedimentary basins (1212); KEYWORDS: black shale formation, depositional environment, Norwegian-Greenland Seaway, Mesozoic, organic petrography Citation: Langrock, U., R. Stein, M. Lipinski, and H.-J. Brumsack, Late Jurassic to Early Cretaceous black shale formation and paleoenvironment in high northern latitudes: Examples from the Norwegian-Greenland Seaway, Paleoceanography, 18(3), 1074, doi:10.1029/2002PA000867, 2003.