Wolfgang Oschmann

The Posidonia Shale (Lower Toarcian) of SW-Germany: an oxygen-depleted ecosystem controlled by sea level and palaeoclimate

Abstract The Lower Toarcian Posidonia Shale is famous for its excellently preserved fossils and its high amount of organic matter (up to 16%). Both quality of preservation and accumulation of organic matter have been explained by permanent anoxic bottom water conditions. High-resolution geochemical, sedimentological and palaeoecological investigations of various sections of the Posidonia Shale in SW-Germany, however, indicate that oxygen availability was variable and ranged from short oxygenated periods to longer-term anoxia. The benthic macrofauna consists of nine fossil communities and was used, in combination with geochemical data, to reconstruct a time-averaged oxygen curve. Anoxic conditions prevailed during the deposition of the Toarcian black shales; they were, however, punctuated by various short periods (weeks to years) with oxygenated bottom water conditions. Sedimentological (e.g. distinctiveness of microlamination, siliciclastic content) and geochemical parameters (e.g. organic matter content, isotopic signatures: δ18O and δ13C, molecular redox parameters: pristane/phytane ratio, arylisoprenoids) exhibit a remarkable covariation and seem to be controlled by sea level fluctuations. Maximum oxygen depletion and an extreme negative shift of δ13Corg values (−34‰) occurred during the early falciferum-zone. This is explained by the recycling of 12C-enriched carbon derived from remineralization of organic matter on and within the substrate during low sea level stand and a highly elevated redox boundary including photic zone anoxia. The subsequent transgression permitted enhanced water exchange with the Tethyan Ocean and caused improvement of living conditions at the end of the falciferum-zone. Other important factors controlling the depositional environment are the overall palaeogeographic situation and climate. The early Jurassic is the latest period before break-up of Pangaea and probably was ruled by a strong meridional atmospheric circulation system with pronounced seasonal changes of prevailing trade- and monsoon-wind systems. An estuarine circulation with a positive water balance and surface water with slightly reduced salinity in the summer alternated with an anti-estuarine circulation and a negative water balance in the winter. During the summer months a stratified water column with anoxic conditions below the halocline developed. δ18O data indicate low salinity in the surface water during the monsoon-influenced summer. High productivity was then located in the photic zone and the corresponding isotopically light δ18O-signal was fixed in the calcareous nannoplankton. During the winter months a saline circulation system brought oxygen to the benthic environment, favouring temporary benthic colonization, especially during times of relative sea level high stand.

Daily Growth Rates in Shells of Arctica islandica: Assessing Sub-seasonal Environmental Controls on a Long-lived Bivalve Mollusk

Abstract Shells of the extremely long-lived bivalve mollusk Arctica islandica (Linnaeus 1767) provide century-long, multi-proxy records of inter-annual environmental variability in middle- to high-latitude marine settings. Reliable interpretation of these climate archives, however, requires exact knowledge of the length and timing of the growing season and which environmental parameters control shell growth rate during the year. Here, intra-annual growth microstructures, δ18O-derived ambient water temperatures, and δ13C from A. islandica shells collected from the southern and central North Sea are studied. Such data were analyzed in conjunction with observational sea-surface temperature and primary productivity data. Arctica islandica produces circadian growth increments in its shell (on average 31.5 μm per day during age four, measured along the outer shell surface), which allow assignment of calendar dates to each shell portion. The growing season of A. islandica in the upper mixed layer of the ocean (here 25 m water depth) is not continuous over an eight-month period as previously suggested. Rather, it is interrupted during spawning between early September and mid-November. In addition, shell production ceases or is strongly retarded due to food scarcity between mid-December and mid-February. Water temperatures derived from oxygen-isotope ratios are in good accord with observed sea-surface temperatures. In specimens at 25 m water depth, abrupt changes in shell δ18O-derived temperature (Tδ18O) were interpreted to represent vertical displacements of the seasonal thermocline. Daily shell growth is controlled by temperature and food availability. Up to 58% of the variation in daily growth rate is explained by these environmental parameters. This study demonstrates that A. islandica can provide seasonal to subseasonal, precisely dated proxies of environmental variables. Such data are of increasing importance for climate models.

North Atlantic Oscillation dynamics recorded in shells of a long-lived bivalve mollusk

Existing reconstructions of the winter North Atlantic Oscillation (WNAO) are based on terrestrial proxies and historical documents. No direct high-resolution, long-term rec ords from marine settings are available for this major climate-dictating phenomenon, which severely affects a variety of economic aspects of our society. Here we present a 245 yr proxy WNAO index based on shells of the long-lived marine bivalve mollusk Arctica islandica . Variations in annual rates of shell growth are positively correlated with WNAO-related changes in the food supply. Maximum amplitudes in frequency bands of 7–9 and 5–7 yr fall exactly within the range of instrumental and other proxy WNAO indices. These estimates were obtained for specimens collected live, 2000 km apart, in the central North Sea and on the Norwegian Shelf. Hence, the WNAO influences hydrographic regimes of large regions of the ocean. Our study demonstrates that A. islandica can reliably reconstruct WNAO dynamics for time intervals and regions without instrumental records. Our new tool functions as a proxy for the WNAO index prior to the twentieth-century greenhouse forcing and has the potential to further validate other proxy-based WNAO records.

Global enhancement of ocean anoxia during Oceanic Anoxic Event 2: A quantitative approach using U isotopes

During the Mesozoic greenhouse world, the oceans underwent several oceanic anoxic events (OAEs) characterized by intervals during which organic-rich black shales were deposited, indicating strong oxygen depletion in the marine realm. The Cenomanian-Turonian OAE2 (ca. 93 Ma) represents one of the most prominent events of the Cretaceous, with significant perturbations of the global carbon cycle. Although OAE2 likely reached a global scale, the spatial extent of seawater anoxia during this OAE is poorly constrained. Here we demonstrate that variations in the 238U/235U isotope ratio (δ238U), a newly developed paleoredox proxy, can be used to quantify the extent of marine anoxia. For black shales from the mid-Cretaceous OAE2 we find a systematic shift toward lighter δ238U and lower U concentrations as compared to modern equivalent organic-rich sediments from the Black Sea. This shift translates to a global increase of oceanic anoxia during OAE2 by at least a factor of three as compared to the present day or to periods before and after OAE2. The constant offset in U concentrations and isotope compositions of black shales throughout OAE2 compared to modern Black Sea sediments indicates an enhancement of oceanic anoxic conditions already prior to the onset of OAE2.

Hardgrounds, reworked concretion levels and condensed horizons in the Jurassic of western India: their significance for basin analysis

Jurassic sediments in the shallow pericratonic basins of Kachchh and Rajasthan, western India, exhibit numerous signs of reduced sedimentation, omission, erosion and in situ reworking, in combination with synsedimentary cementation. Hardgrounds developed on carbonate shoals in the Bathonian of Rajasthan, whilst reworked concretion levels are characteristic of offshore siliciclastic sediments of the Callovian of Kachchh. A prominent marker horizon, the Oxfordian Dhosa Oolite Member, occurs throughout much of the Kachchh sub-basin and is a highly condensed unit characterized by hardgrounds, intraformational cobbles, reworked concretions, stromatolitic iron crusts, iron oncoids, and shell lags. Hardgrounds, reworked concretion levels, and condensation horizons are interpreted as the preserved relicts of transgressive pulses. Such pulses were possibly controlled by eustatic rise of sea level. Of at least equal importance, however, was a tectonic control which is demonstrated by the presence of small neptunian dykes, boulder beds derived from small submarine cliffs and rapid lateral facies and thickness changes in the Dhosa Oolite Member. These indications of extensional tectonics are thought to be connected to rifting and initial sea floor spreading between Africa and India.

Kimmeridge clay sedimentation — A new cyclic model

Abstract The Kimmeridgian (Upper Jurassic) fine-clastic sequences of the Boulonnais (northern France) and Dorset (southern England) represent oxygen controlled environments. Five facies types with increasingly deteriorating living conditions, ranging from normal shelf environments to submarine lifeless desert, have been distinguished. The widespread occurrence of this facies in the Late Jurassic North Atlantic Shelf Sea and the palaeogeographic situation require a large-scale model to explain the Kimmeridge Clay sedimentation. Anoxic conditions are generally explained by one of three models, none of which applies directly to the Kimmeridge Clay. A new model, derived from the overall palaeogeographic and palaeoclimatic situation is therefore proposed. In this model, water mass stratification and anoxic bottom conditions refer to a high latitude monsoonal type wind circulation causing a water current and counter current system in the North Atlantic Shelf Sea and the Arctic Ocean. Variations in the amount of oxygen in the bottom waters allow recognition of four different cycles with periods ranging from seasonal to 3 × 10 7 years. They influenced the counter current system and the resulting anoxic conditions.

An Early Carboniferous seep community and hydrocarbon-derived carbonates from the Harz Mountains, Germany

Early Carboniferous (latest Visean) seep deposits occur on top of the drowned Middle Devonian–Late Devonian Iberg atoll reef, Harz Mountains, Germany. These deposits include limestone with a low-diversity but high-abundance fauna of rhynchonellid brachiopods and rare solemyid bivalves, as well as microbial limestone. Rhynchonellids form dense, autochthonous shell accumulations and are generally articulated. They are closely associated with hydrocarbon-derived carbonates. The carbonates exhibit δ 13 C values as low as −32‰, relative to the Peedee belemnite standard, revealing that they are predominantly hydrocarbon derived. The fauna, carbonate fabrics, and isotope signatures provide unequivocal evidence for a seep origin of the Visean deposit. The occurrence of solemyid bivalves supports this interpretation as members of this family (1) are well known for their relationship with chemoautotrophic bacteria and (2) have been reported from ancient and modern seeps. Possible hydrocarbon sources are thermogenic methane derived from the volcanic base of the Iberg reef or methane from a petroleum reservoir.

Palaeoenvironmental reconstruction of Lower Toarcian epicontinental black shales (Posidonia Shale, SW Germany): global versus regional control

Abstract A detailed multidisciplinary investigation (sedimentology, palaeoecology, geochemistry) of the Lower Toarcian Posidonia Shale revealed that the depositional environment was mainly controlled by sea level changes and palaeoclimate. Carbon isotope values of both, carbonates and organic matter are closely related to environmental conditions. Highly unfavourable living conditions for benthic fauna prevailed during a relative sea level low stand resulting in an enclosed stagnant basin environment. Long-term benthic colonization could not occur until water circulation improved during sea level high stand. The carbon isotope record of the Posidonia Shale is primarily controlled by redox conditions. Due to the dependency of the isotopic composition on regional palaeoecological, sedimentological and geochemical conditions, on oxygen availability and sea level changes, there is no need to infer global atmospheric changes of pCO2 or oceanwide anoxic events.

Faunal response to transgressive−regressive cycles : example from the Jurassic of western India

Abstract Upper Callovian to Oxfordian shelf sediments at Ler in the Kachchh Basin, western India, are interpreted in terms of transgressive-regressive cycles. The transgressive phases are represented by thin layers of reworked and bored concretions, sometimes in association with skeletal concentrations, the regressive phases are documented by much thicker units of largely fine-grained sediments. The benthic fauna of transgressive and regressive phases differes markedly and thus mirrors the sedimentary cycles: During transgressive phases non-sedimentation produced hard substrates colonized mainly by byssate, cemented, or pedicle-attached suspension-feeding epifaunal species, whilst sediment input during regressive phases lead to soft substrate conditions characterized by infaunal deposit- and suspension-feeders as well as by epifaunal opportunists. Whereas the composition of the “regressive fauna” remains fairly constant through time, that of successive “transgressive faunas” often differs drastically. This is possibly due to a greater environmental sensitivity of epifaunal taxa as compared to infaunal taxa.

A 1500-Year Holocene Caribbean Climate Archive from the Blue Hole, Lighthouse Reef, Belize

Abstract Sediment cores (up to 6 m in length) from the bottom of the Blue Hole, a 125 m deep Pleistocene sinkhole located in the lagoon of Lighthouse Reef Atoll, Belize, consist of undisturbed, annually layered biogenic carbonate muds and silts with intercalated coarser grained storm beds. The sedimentation rate of the layered sections is 2.5 mm/y on average, and the long cores span the past 1500 years. Oxygen isotopes of laminated sediment provide a late Holocene climate proxy: A high-resolution δ18O time series traces the final Migration Period Pessimum, the Medieval Warm Period, the Little Ice Age, and the subsequent temperature rise. Carbon isotopes (δ13C) decrease up core and show the impacts of the decline of the Mayan culture and the Suess effect. Time series analyses of δ18O and δ13C content reveal 88-, 60-, 52-, and 32-year cyclicities, and suggest solar forcing. Storm event beds are most common during AD 650–850, around AD 1000, during AD 1200–1300, and AD 1450–1550. Major storm beds are rare during the past 500 years BP.