Eddy Keppens

High resolution distribution of trace elements in the calcite shell layer of modern mytilus edulis: environmental and biological controls

Abstract Mussels of a similar size, originating from the same population, have been grown in the field and the high resolution distribution of Mg, Mn, Sr, Ba and Pb in their calcite shell layer, as determined by Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry, has been compared to temporal variations of environmental parameters. All elements exhibit cyclic variations with an annual periodicity. The Mg, Sr and Pb cycles show great similarity and are characterised by a broad maximum during spring and early summer. These profiles cannot be explained by seasonal variations in the seawater composition. Skeletal Mg covaries reproducibly with temperature during spring but this covariation is abruptly interrupted after the spring phytoplankton bloom. The absence of a constant Mg-temperature relationship over the year hampers the direct use of Mg in M. edulis calcite as a high resolution temperature proxy. The sharp peak that is dominating each skeletal Ba cycle coincides with the annual algal biomass maximum and presumably reflects elevated concentrations of particulate Ba, associated with the spring phytoplankton bloom. Similarly, elevated skeletal Mn concentrations during spring might reflect bloom-induced increases in particulate Mn. Skeletal δ 13 C shows a seasonal variation, characterised by a minimum that coincides with the Mn maximum. These δ 13 C variations are not in equilibrium with the seasonal δ 13 C trend of the seawater dissolved inorganic carbon and presumably reflect fluctuations in the contribution of metabolic carbon to the shell carbonate, corresponding to seasonal variations in the mussel’s respiration rate.

Holocene climate variability in Europe: Evidence from δ18O, textural and extension-rate variations in three speleothems

Time-series O isotope profiles for three U–Th dated stalagmites have revealed that for much of the Holocene, a site on the Atlantic seaboard (SW Ireland) exhibits first-order δ18O trends that are almost exactly out of phase with coupled δ18O curves from two southern European sites (SE France and NW Italy). In the Irish stalagmite (CC3 from Crag Cave, SW Ireland), low δ18O at 10,000 cal yr BP reflects cool conditions. By the early to mid-Holocene (9000–6000 cal yr BP) δ18O had increased, reflecting the onset of warmer conditions on the Atlantic seaboard. This shift to higher δ18O was accompanied by a marked increase in the stalagmite extension rate, reinforcing our interpretation that this was a period of relative warmth. Except for an episode of increased extension rate about 5500 yr ago, δ18O in the Crag stalagmite exhibits a gradual decrease, accompanied by declining extension rates between 7800 and 3500 cal yr BP, interpreted as a cooling trend. There is evidence for increases in both δ18O and stalagmite extension rate in the period from 3500 cal yr BP to the present day suggesting a return to warmer conditions on the Atlantic seaboard. In the stalagmite from NW Italy (ER76, Grotta di Ernesto, Trentino province) the early-Holocene (c. 9200-7800 cal yr BP) is characterised by high δ18O, probably indicative of warm and/or dry conditions. Exceptionally low δ18O from 7800 to 6900 cal yr BP at this site reflects a well-defined wet phase (Cerin wet phase). In the last three millennia, this stalagmite exhibits a shift to lower δ18O, interpreted as some combination of cooler and/or wetter conditions. Unlike the Irish stalagmite, the Italian sample does not show a correlation between δ18O and extension rate. Instead, its extension rate correlates roughly with δ13C, presumably reflecting a climate-driven vegetation change. In the early Holocene, δ18O in the French stalagmite (CL26, Grotte de Clamouse, Herault province, SE France) was low relative to its Holocene average. For much of the period since c. 3500 cal yr BP this stalagmite exhibits higher δ18O than in the early Holocene, suggesting warmer conditions. Like the Irish stalagmite, the French sample exhibits a well-defined correlation between δ18O and extension rate. Had drip-water availability been the dominant control on δ18O at this semi-arid site then higher δ18O would have been accompanied by lower, not higher extension rates. This suggests strongly that temperature rather than rainfall amount was the dominant control at this site. While conclusions regarding the patterns of climate variability on a continent scale must remain tentative because of the limited number of stalagmites studied we argue that early Holocene warm conditions on the Atlantic seaboard (Irish site) coincided with relatively cool conditions at the Clamouse site. By c. 3500 yr ago the pattern appears to have been reversed.

Origin and fate of organic carbon in the freshwater part of the Scheldt Estuary as traced by stable carbon isotope composition

We investigated the seasonal and geographical variation in the stable carbon isotope ratios of total dissolved inorganic carbon (δ13 cdic) and suspended matter(δ13 CPOC) in the freshwater part of the River Scheldt. Two major sources of particulate organic matter (POM) occur in this riverine system: riverine phytoplankton and terrestrial detritus. In winter the lowest δ13 cdic values are observed due to enhanced input of CO2 from decomposition of13C-depleted terrestrial plant detritus (average δ13Cdic=-14.3‰). During summer, when litter input from terrestrial flora is the lowest, water column respiration on POM of terrestrial origin is also the lowest as evidenced by less negative δ13Cdic values (average δ13Cdic =−9.9‰). In winter the phytoplankton biomass is low, as indicated by low chlorophyll a concentrations (Chl a < 4.5 μgl−1), compared to summer when chlorophyll a concentrations can rise to a maximum of 54 μgl−1. Furthermore, in winter the very narrow range of δ13 Cpoc (from -26.5 to -27.6‰) is associated with relatively high C/N ratios (C/N > 9) suggesting that in winter a major fraction of POC is derived from allochthonous matter. In summer δ13 cpoc exhibits a very wide range of values, with the most negative values coinciding with high Chl a concentrations and low C/N ratios (C/N< 8). This suggests predominance of phytoplankton carbon in the total particulate carbon pool, utilising a dissolved inorganic carbon reservoir, which is already significantly depleted in13C. Using a simple two source mixing approach a reconstruction of the relative importance of phytoplankton to the total POC pool and of13C/12C fractionation by phytoplankton is attempted.

BIOLOGICAL AND DIAGENETIC INFLUENCE IN RECENT AND FOSSIL TUFA DEPOSITS FROM BELGIUM

Abstract Both Recent and fossil tufa deposits from Belgium were studied by petrographical and geochemical means. Study of the Recent deposits provides information on precipitation conditions and on possible present-day biological influences of organisms on tufa precipitation. Comparative petrographical and geochemical investigations of both Recent and fossil deposits give information about diagenetic modifications. The Recent deposits form within small rivers, where they display two macro-types, i.e. precipitates on organic substrates (plants, mosses, algae, cyanobacteria) and laminated carbonate crusts. The latter occur on the river bed or on different substrates (limestone rocks, snail shells, artificial glass and limestone plates). Characteristic petrographical features are, on the one hand, horizontal laminations within the carbonate crusts, which relate to seasonal growth patterns of the present cyanobacteria and on the other hand, the close association between the organic matter and carbonate precipitates in the actively forming deposits. Due to encrustation in growth position, the organic structures determine the overall structure of the precipitates. Characteristic crystal morphologies (pseudo-hexagonal crystals and penta-dodecahedral crystals on mosses and algae) and specific features (e.g. micritic cyanobacterial bushes) show possible direct influence of the organisms on calcite precipitation. The stable carbon isotope signature of the Recent deposits with a mean δ13C value of about −11.05‰ PDB points to a mixture of three possible CO2 sources, i.e. atmospheric-related CO2, recycled marine CO2 and biological CO2. The studied fossil deposits consist of granular detrital tufa and porous cascade tufa with precipitates on organic substrates and laminated carbonate crusts, which show similar characteristics to the macrotypes found in the Recent deposits. However, there are differences between Recent and paleo-deposits that can be attributed to diagenetic changes. Moulds and prints within the tufas are created by disappearance of the original organic matter. The fossil deposits are also characterized by the presence of sparry crusts that originate from cementation and recrystallization of initial micritic cyanobacterial bushes. Furthermore, specific calcite crystals such as euhedral, gothic arch, needle and needle fibre calcite crystals have been recognized. These are interpreted to have formed during phreatic and vadose diagenesis. The stable carbon and oxygen isotope compositions of the fossil deposits show a shift towards slightly less depleted values and smaller variation with respect to the Recent deposits. This feature is also interpreted in terms of diagenetic changes within the tufa deposits.

Mg, Sr and Sr isotope geochemistry of a Belgian Holocene speleothem: Implications for paleoclimate reconstructions

In this study, variations in Mg/Ca, Sr/Ca and 87Sr/86Sr ratios in a Holocene Belgian speleothem (cave secondary carbonate deposit) are interpreted in terms of changes in water residence time and changes in weathering processes, possibly induced by changes in West European climate. A stalagmite from the Pere Noel cave (Belgium) was dated with the TIMS U/Th method and was deposited between ~ 13 and ~ 2 ka BP. The 1000 Mg/Ca ratio varies between 4.9 and 26, and displays short-term changes but no significant long-term trend. The 1000 Sr/Ca ratio varies between 0.09 and 0.31 and displays both short-term changes and a long-term decreasing trend from 12.9 to 3.5 ka. In parallel, the Sr isotopic composition of the speleothem decreases from 0.7090 at 12.9 ka to 0.7088 at 3.5 ka. The Sr isotope ratio is higher than expected from the overlying limestone (87Sr/86Sr = 0.7081) which implies an external source of radiogenic Sr, most probably from a silicate phase. Short-term and long-term changes in Mg/Ca, Sr/Ca as well as long-term changes of 87Sr/86Sr ratios can be explained by changes in the dissolution and precipitation processes of the host limestone. These processes are controlled by changes in water residence times linked to changes in the water excess (precipitation minus evapo-transpiration)

Sclerosponges as a new potential recorder of environmental changes: Lead in Ceratoporella nicholsoni

Lead concentrations have been analyzed on a 223 yr profile through the aragonitic skeleton of the reef-building Caribbean sclerosponge Ceratoporella nicholsoni by using laser-ablation inductively coupled plasma‐mass spectrometry. A parallel study of the δ 13 C distribution in the skeleton validates the previously established mean annual growth rate of 230 µm/yr, at least for long-term important environmental changes. The Pb trend in the specimen displays a general increase from 0.30 ppm ca. A.D. 1760 to 2.15 ppm ca. A.D. 1984; a major threefold increase occurred after 1930. This Pb profile is analogous to results acquired from ice or coral cores and clearly highlights the potential of sclerosponges as a new proxy of environmental changes for time series extending over several centuries.

Geochemical constraints on the origin and migration of palaeofluids at the northern margin of the Variscan foreland, southern Belgium

Three major fracture types in the Dinantian of the Namur syncline at the northern margin of the Variscan foreland in southern Belgium, have been investigated by fluid inclusion and stable isotope analysis. The oldest and volumetrically most important fracture type is characterized by conjugated and sigmoidal en-echelon calcite veins which formed during the Variscan folding. These veins, and the surrounding limestones, have both a similar dull brown-orange luminescence and stable isotopic composition (δ18O = −11ℵ. to −8‰ PDB and δ13C = 0ℵ. to + 3‰ PDB). This indicates precipitation of the calcite cement from a fluid buffered by the rock. In the area studied, only a limited amount of fluids was expelled through the Dinantian during the main phase of Variscan compressional tectonism. Fractures filled with ferroan calcites cross-cut the Variscan .folds. Fluid inclusion and stable isotopic evidence indicates that the calcites precipitated at 40°–60°C from a saline fluid (9.2 to 23.2 eq. wt% NaCl) with an estimated δ18O composition between −2.0‰ and −0.6ℵ. SMOW. The most likely origin of these high-salinity fluids with a relatively low-oxygen isotopic composition is a gravity-driven meteoric water which underwent an intense water-rock interaction. The topographic relief created by the Variscan tectonism could have allowed groundwater to penetrate into the deeper subsurface and to migrate towards the margin of the foreland basin. Non-ferroan, fracture-filling calcites formed later during the Mesozoic or Tertiary. Low-salinity meteoric waters and high-salinity CaCl2NaCl brines with a temperature around 50°C migrated through this fracture system. Along these fractures, meteoric water migrated down into the subsurface and the brines could have flowed upward from the basement.

Oxygen and carbon isotope study of the Holocene oyster reefs and paleoenvironmental reconstruction on the northwest coast of Bohai Bay, China

Abstract The northwest coast of Bohai Bay supports large areas of Holocene Crassostrea gigas (oyster) reefs. Their huge size and the fact that the shells are composed of consecutive microgrowth layers, consisting of primary calcite, make them an ideal object for a stable isotope study. The δ18O and δ13C compositions of the oysters reveal a number of interesting features. The most important are annual cycles related to seasonal variations, the regional δ18O (water) versus salinity ratio, and a stop in shell secretion below a water temperature of 11.5 ± 1.5 °C. A positive correlation (covariance) between δ18O and δ13C values is present in most specimens. The δ18O and δ13C values of single specimens are believed to be mainly controlled by water temperature and the occurrence of phytoplankton blooms and decomposition. However, correlation among oysters of different ages indicate that the δ18O and δ13C values are mainly dependant of three variables: water temperature, salinity and phytoplankton blooms/decomposition. Between ca. 6500 yrs cal B.P. and 2300 yrs cal B.P. the salinity in Bohai Bay increased from less than 20%. to 30%.. Based on geological data combined with ecological data from C. gigas, we suggest that precipitation decreased during this period, while the input of fluvial sediments increased. The sediment load of rivers increased markedly after ca. 4000 yrs cal B.P., possibly due to a change in climate. In our study area this resulted in the development of the Oyster Plain.

Sr isotopic constraints on the process of glauconitization

Sr isotopic data of Holocene glauconitic pellets suggest a twofold glauconitization process that is strongly linked to the detrital clay minerals from surrounding mud which act as precursors until the K2O content of the glauconitic material reaches 4%. Above that, a detectable sea-water influence becomes progressively predominant. The process seems to start with a dissolution-crystallization reaction in equilibrium with the precursor, which is followed by a maturation process in isotopic equilibrium with sea-water Sr, while the K2O content increases further. This chemical evolutionary process may also occur for other types of clay minerals.

Origin and fate of dissolved inorganic carbon ininterstitial waters of two freshwater intertidalareas: A case study of the Scheldt Estuary, Belgium

Processes affecting the concentration and isotopiccomposition of dissolved inorganic carbon (DIC) wereinvestigated in pore waters of two freshwaterintertidal areas of the Scheldt Estuary, Belgium. Porewater δ13CDIC values from marshes andmudflats varied from −27 to +13.4‰, these very largevariations reflect the contribution of differentcarbon sources to the DIC pool.In pore waters of the upper mudflat, river water DICand dissolution of calcite contribute to a lesserextent (10% and 16% respectively) to the total DICpool. Results indicate that inorganic carbon added tothe pore water of the mudflats has a δ13Cvalue of +20.3‰ in May 1998. These strongly enrichedδ13CDIC values suggest that the majorcontribution (up to three-quarters) to total DIC isCO2 derived from methanogenesis.In pore waters of the marshes, CO2 derived fromorganic matter degradation (−27.5‰) and river DIC(−11.5 to −16.1‰) are the major sources of inorganiccarbon contribution to the total DIC pool. In porewaters from a marsh site colonised by willow trees,the contribution from CO2 derived from organicmatter degradation is larger than in pore waters froman area with only reed vegetation. In the latter caseriver water DIC is the major source of pore waterDIC.