Judith A. McKenzie

CO2 control on carbon-isotope fractionation during aqueous photosynthesis: A paleo-pCO2 barometer

A carbon-isotope study of the various carbon phases during an annual productivity cycle in eutrophic Lake Greifen, Switzerland, indicates that values for the photosynthetic isotopic fractionation (ϵp) between dissolved CO2 and particulate organic carbon (POC) and δ13CPOC increase in response to decreasing concentrations of dissolved carbon dioxide ([CO2(aq)]) due to increased carbon demand during photosynthesis. Seasonal changes in the dominant biological assemblage may also be related to [CO2(aq)] and may influence the fractionation as a consequence of changes in the dominant photosynthetic mechanism (CO2 vs. HCO3- Utilization). A logarithmic relation between ϵp and [CO2(aq)] may be useful for evaluating partial pressures of paleoatmospheric CO2.

Origin of calcite in syntectonic veins as determined from carbon-isotope ratios

A particular feature of many Alpine tectonic thrust sheets composed of carbonates is the development of distinct generations of calcite-filled veins. In the Doldenhorn nappe, one of the Helvetic nappes of Switzerland, we were able to distinguish five different generations of calcite-filled veins. Determination of carbon-isotope ratios in these calcite veins and their adjacent limestone matrix indicates that the earlier vein systems contain carbonate material not in isotopic equilibrium with the adjacent matrix. In the case of the later vein systems, the carbon-13 content approaches that of the nearby matrix. The isotope data indicate a transition from a chemically open system to a closed system; this trend is related to an overall modification of the deformation mechanism of the nappe, resulting in the increasing importance of pressure solution with time.

The role of sea-surface temperature as a control on carbonate platform development in the western Coral Sea

The derivation of a detailed sea-surface paleotemperature curve for the middle Miocene-Holocene (10-0 Ma) from ODP Site 811 on the Queensland Plateau, northeast Australia, has clarified the role of sea-surface temperature fluctuations as a control on the initiation and development of the extensive carbonate platforms of this region. This curve was derived from isotopic analyses of the planktonic foraminifer Globigerinoides ruber, and converted to temperature using the surface-water paleotemperature equation accounting for variations in global ice volume. The accuracy of these data were confirmed by derivation of paleotemperatures using the water column isotopic gradient (Δδ13O), corrected for salinity and variations in seafloor water mass temperature. Results indicate that during this period surface-water temperatures were, on average, greater than the minimum required for tropical reef growth (20°C; Veron, 1986), with the exception of the late Miocene and earliest early Pliocene (10-4.9 Ma), when there were repeated intervals of temperatures between 18–20°C. Tropical reef growth on the Queensland Plateau was extensive from the early to early middle Miocene (∼21-13 Ma), after which reef development began to decline. A lowstand near 11 Ma probably exposed shallower portions of the plateau; after re-immersion near 7 Ma, the areal extent of reef development was greatly reduced (∼50%). Paleotemperature data from Site 811 indicate that decreased sea-surface temperatures were likely to have been instrumental in reducing the area of active reef growth on the Queensland Plateau. Reduced reefal growth rates continued until the late Pliocene or Quaternary, despite the increase of average sea-surface paleotemperatures to 22–23°C. Studies on modern corals show that when sea-surface temperatures are below ∼24°C, as they were from the late Miocene to the Pleistocene off northeast Australia, corals are stressed and growth rates are greatly reduced. Consequently, when temperatures are in this range, corals have difficulty keeping pace with subsidence and changing environmental factors. In the late Pliocene, sedimentation rates increased due to increases in non-reefal carbonate production and falling sea levels. It was not until the mid-Quaternary (0.6–0.7 Ma) that sea-surface paleotemperatures increased above 24°C as a result of the formation of a western Coral Sea warm water pool. Because of age discrepancies, it is unclear exactly when an effective barrier developed on the central Great Barrier Reef; the formation of the warm water pool was likely to have either assisted the formation of this barrier and/or permitted increased coral growth rates. Fluctuations in sea-surface temperature can account for much of the observed spatial and temporal variations of reef growth and carbonate platform distribution off northeast Australia, and therefore we conclude that paleotemperature variations are a critical control on the development of carbonate platforms, and must be considered an important cause of ancient platform “drowning”.

Abu Dhabi sabkha, Persian Gulf, revisited: Application of strontium isotopes to test an early dolomitization model

A study of brines and diagenetically altered minerals from coastal sabkha of Abu Dhabi, Persian Gulf, was undertaken to test the flood recharge-evaporative pumping model for early dolomitization using strontium isotopes as a tracer. The {sup 87}Sr/{sup 86}Sr ratio of the sabkha brines defines a pattern of both horizontal and vertical mixing of coastal seawater and continental ground water consistent with a previously proposed model. The {sup 87}Sr/{sup 86}Sr ratio of the diagenetic dolomites indicates early dolomitization by marine recharge waters. The {sup 87}Sr/{sup 86}Sr ratio of gypsum from the dolomitization area (1.4 to 4.4 km from high-tide mark) and seaward reflects precipitation from marine brines, but more landward (6 to 9 km), gypsum and anhydrite record a continental signal in the minerals suggests a paragenetic evaporative sequence (dolomite {yields} gypsum {yields} anhydrite). The dolomitizing fluid is evaporated seawater that recharges the sabkha aquifer during supratidal flooding and is afterward pumped upward through the saturated zone by evaporative processes.

Oxygen isotopic composition of the Mediterranean Sea since the Last Glacial Maximum: constraints from pore water analyses

Abstract Interstitial waters recovered from Ocean Drilling Program, Leg 161, site 976 in the western Mediterranean Sea are used in conjunction with a numerical model to constrain the δ18O of seawater in the basin since the Last Glacial Maximum, including Sapropel Event 1. To resolve the oxygen isotopic composition of the deep Mediterranean, we use a model that couples fluid diffusion with advective transport, thus producing a profile of seawater δ18O variability that is unaffected by glacial–interglacial variations in marine temperature. Comparing our reconstructed seawater δ18O to recent determinations of 1.0‰ for the mean ocean change in glacial–interglacial δ18O due to the expansion of global ice volume, we calculate an additional 0.2‰ increase in Mediterranean δ18O caused by local evaporative enrichment. This estimate of δ18O change, due to salinity variability, is smaller than previous studies have proposed and demonstrates that Mediterranean records of foraminiferal calcite δ18O from the last glacial period include a strong temperature component. Paleotemperatures determined in combination with a stacked record of foraminiferal calcite depict almost 9°C of regional cooling for the Last Glacial Maximum. Model results suggest a decrease of ∼1.1‰ in seawater δ18O relative to the modern value caused by increased freshwater input and reduced salinity accompanying the formation of the most recent sapropel. The results additionally indicate the existence of isotopically light water circulating down to bottom water depths, at least in the western Mediterranean, supporting the existence of an ‘anti-estuarine’ thermohaline circulation pattern during Sapropel Event 1.

Stable-isotope mapping in Messinian evaporative carbonates of central Sicily

Mapping stable-isotope isolines in both modern and ancient evaporative carbonate formations demonstrates that regional variations of δ 18 O values exist. In the case of the Messinian Central Sicilian Basin, an ancient evaporite basin, the distribution pattern of the δ 18 O isolines can be related to the paleogeometry and paleohydrology of the basin. Isolines of equal δ 13 C values also show large regional variations possibly reflecting the domination of different bacterial processes (oxidation, sulfate reduction, or methanogenesis) in different regions. The potential application for stable-isotope mapping in evaporative carbonate formations deposited in both marine and lacustrine environments lies in its ability to predict the location of depositional centers in ancient evaporite basins.

A 600-year sedimentary record of flood events from two sub-alpine lakes (Schwendiseen, Northeastern Switzerland)

Short sediment cores from two small interconnected sub-alpine lakes (Vorderer and Hinterer Schwendisee) in northeastern Switzerland contain a continuous record of environmental changes, which occurred during the last 600 years. Several light-colored layers within predominantly organic carbon-rich sediments display elevated density values and a high amount of detrital material. This composition points towards an allochthonous origin, which we interpret as material brought into the lake by unusually strong rainfall events. Based on this interpretation, four event horizons were identified in the sedimentary record. These horizons were dated combining a 137Cs and 210Pb-based chronology with meteorological and historical data. They occurred mostly within the 19th and at the beginning of the 20th century, whereas only one event occurred prior to 1800. The well-documented strong rainfall event that occurred on June 14, 1910 is observed in the sedimentary record as the thickest detrital horizon. These short-lived climatic events archived in the sediments of both lakes are superimposed on a longer-term environmental trend characterized by fluctuating conditions in organic productivity. The formation and preservation of authigenic minerals, such as siderite and vivianite, are most likely related to intervals of different primary productivity in the water column.

Numerical ages of Cenozoic biostratigraphic datum levels: Results of South Atlantic Leg 73 drilling

Six sites were drilled in the South Atlantic during the Leg 73 cruise of the Deep Sea Drilling Project (DSDP). Hydraulic piston coring at five of the six sites obtained a nearly complete sequence of undisturbed Cenozoic samples. The magnetostratigraphy at those sites was investigated by close sampling representing time intervals of about 10 4 yr. Most of the Cenozoic nannofossil and many of the foraminiferal zonal boundaries were accurately determined and magnetostratigraphically calibrated at those five Leg 73 boreholes. Their numerical ages have been computed assuming a linear spreading rate and a radiometric age of 66.5 m.y. for the Cretaceous-Tertiary boundary. Alternative magnetostratigraphic ages were obtained with the adoption of a 63.5-m.y. age for the Cenozoic. Our data confirm previous determinations of the Pleistocene-Pliocene boundary at 1.8 (1.7) m.y. B.P. and of the Pliocene-Miocene boundary at 5.1 (5.0) m.y. B.P. The Miocene-Oligocene boundary is placed within chron C-6 C, with a magnetostratigraphic age of 23.8–24.0 (22.7–22.9) m.y. The Oligocene-Eocene age is also very precisely located within chron C-13 R, with a magnetostratigraphic age of 37.1–37.2 (35.5–35.6) m.y. The Eocene-Paleocene boundary should be located within an uncored interval of chron C-24, with a magnetostratigraphic age of 59.0 (55.4) ± 0.2 m.y. A general accord of the magnetostratigraphic ages from Leg 73 sites and the radiometric ages published in the literature for the various zonal boundaries validates the assumption of a linear sea-floor–spreading rate during the Cenozoic.

Geochemical variations in Quaternary hardgrounds from the Hellenic Trench region and possible relationship to their tectonic setting

Abstract The progressive lithification of Quaternary hardgrounds forming in the Hellenic Trench region of the eastern Mediterranean is documented by increasing 18O values and high-magnesium calcite contents, which reflect cold, penesaline waters. The presence of aragonite and dolomite in hardgrounds from the Strabo Trench, which are also enriched in 12C, indicates that the diagenetic solutions vary locally. The origin of this unusual mineralogy and carbon isotopic content could be explained by the upward migration of brines from the underlying Messinian evaporites and the subsurface oxidation of 12C enriched organic material. Shallow burial of the evaporites in the Strabo Trench, a region dominated by transcurrent motion, as opposed to deeper burial in the Matapan- and Poseidon Trench, regions dominated by subduction and thrusting motion, could be responsible for geochemical variations in the diagenetic solutions.