Description: Mg/Ca and Sr/Ca ratios were determined on a single species of planktonic foraminiferan, Globigerinoides ruber (white), collected from the Gulf of Eilat and cultured in seawater at five different salinities (32 to 44), five temperatures (18 to 30 °C) and four pH values (7.9 to 8.4). The Mg/Ca-temperature calibration of cultured G. ruber (with an exponential slope of 8 ± 3%/°C) agrees well with previously published calibrations from core-tops and sediment traps. However, the dependence of Mg/Ca on salinity (with an exponential slope of 5 ± 3%/psu) is also significant and should be included in the calibration equation. With this purpose, we calculated a calibration equation for G. ruber dependent on both temperature and salinity within the 95% confidence limits: Mg/Ca(mmol/mol)=exp[0.06(±0.02)*S(psu)+0.08(±0.02)*T(°C)−2.8(±1.0)],R2=0.95 The influence of pH on Mg/Ca ratios is negligible at ambient seawater pH (8.1 to 8.3). However, we observe a dominating pH control on shell Mg/Ca when the pH of seawater is lower than 8.0. Sr/Ca in G. ruber shows a significant positive correlation with average growth rate. Presumably, part of the variability in shell Sr/Ca in the geological record is linked to changes in growth rates of foraminifera as a response to changing environmental conditions.

Description: We investigated the controls of hydrography and of scavenging on the distribution of the particle reactive radionuclides 231Pa and 230Th in the water column and in surface sediments off Southwest Africa (Angola and Cape basins). Based on a vertical section of total 230Thex concentrations in the water column we show that small differences in the salinity between the North Atlantic Deep Water (NADW) in the Angola Basin and the NADW in the Cape Basin as well as the advection of NADW associated with the Namib Col Current are reflected in total 230Thex concentrations. These variable total concentrations are believed to reflect the flow path and mixing history of NADW with the NADW in the Angola Basin being relatively older and 230Th enriched compared to the NADW in the Cape Basin. In the area investigated we found high 231Paex/230Thex ratios (231Paex/230Thex 〉 0.093) in surface sediments at the continental margin and lower ones (231Paex/230Thex 〈 0.093) in the open ocean. Such a distribution is normally interpreted to result from high particle flux at ocean margins (boundary scavenging). However, the lack of any significant depletion of dissolved 230Th and 231Pa in the water column does not indicate extensive scavenging at the continental margin. High 231Paex/230Thex ratios are constrained to shallow waters depths (〈 2000 m) only and coincide with low fractionation between 231Pa and 230Th indicating that preferential scavenging of 231Pa on opal may have caused high 231Paex/230Thex ratios in the sediments. The observed close negative correlation (r2 = 0.82) between 231Paex/230Thex ratios in sediments and water depths is believed to reflect changes in the particle composition, i.e. a decrease in opal content with water depth. In the Angola and Cape basins the total 231Paex concentrations in NADW were the highest observed so far in the Atlantic Ocean, and they are attributed to the meridional export of 231Pa from the North Atlantic. This caused the average dissolved 231Pa/230Th in the Southeast Atlantic to be about a factor 2 higher when compared to the North Atlantic (Labrador Sea). These differences in the dissolved 231Pa/230Th were not reflected in 231Pa/230Th ratios of surface sediments because the fractionation is lower in the Labrador Sea compared to the Southeast Atlantic, i.e. fractionation counteracts changes in the dissolved 231Pa/230Th. This suggests that fractionation is more important for the determination of 231Paex/230Thex ratios in sediments than the meridional export of 231Pa from the North Atlantic.

Description: The weathering of silicate minerals exposed on the continents is the largest sink of atmospheric CO2 on time scales of millions of years. The rate of this process is positively correlated with global mean temperature and atmospheric CO2 concentration, resulting in a negative feedback that stabilizes Earths’ climate (Berner, 2004). Detrital silicates derived from the physical denudation of the continents are a major component of marine sediments (Li and Schoonmaker, 2003). However, their geochemical behaviour is poorly understood and they are considered to be unimportant to the long-term carbon cycle. We show that in organic matter-rich sediments of the Sea of Okhotsk detrital silicates undergo intense weathering. This process is likely favoured by microbial activity, which lowers pore water pH and releases dissolved humic substances, and by the freshness of detrital silicates which originate from the cold, poorly weathered Amur River basin. Numerical simulations of early diagenesis show that submarine weathering rates in our study area are comparable to average continental weathering rates (Gaillardet et al., 1999). Furthermore, silicate weathering seems to be widespread in organic matter-rich sediments of continental margins, suggesting the existence of a significant CO2 sink there. These findings imply a greater efficiency of the silicate weathering engine also at low surface temperatures, resulting in a weakening of the negative feedback between pCO2, climate evolution and silicate weathering.

Description: ODP Leg 204, which drilled at Hydrate Ridge, provides unique insights into the fluid regime of an accretionary complex and delineates specific sub-seafloor pathways for fluid transport. Compaction and dewatering due to smectite–illite transition increase with distance from the toe of the accretionary prism and bring up fluids from deep within the accretionary complex to sampled depths (≤ 600 mbsf). These fluids have a distinctly non-radiogenic strontium isotope signature indicating reaction with the oceanic basement. Boron isotopes are also consistent with a deep fluid source that has been modified by desorption of heavy boron as clay minerals change from smectite to illite. One of three major horizons serves as conduit for the transport of mainly fluid. Our results enable us to evaluate fluid migration pathways that play important roles on massive gas hydrate accumulations and seepage of methane-rich fluids on southern Hydrate Ridge.

Description: Exploring the potentials of new methods in palaeothermometry is essential to improve our understanding of past climate change. Here, we present a refinement of the published δ44/40Ca-temperature calibration investigating modern specimens of planktonic foraminifera Globigerinoides sacculifer and apply this to sea surface temperature (SST) reconstructions over the last two glacial–interglacial cycles. Reproduced measurements of modern G. sacculifer collected from surface waters describe a linear relationship for the investigated temperature range (19.0–28.5 °C): δ44/40Ca [‰] = 0.22 (±0.05)∗SST [°C] −4.88. Thus a change of δ44/40Ca[‰] of 0.22 (±0.05) corresponds to a relative change of 1 °C. The refined δ44/40Camodern-calibration allows the determination of both relative temperature changes and absolute temperatures in the past. This δ44/40Camodern-calibration for G. sacculifer has been applied to the tropical East Atlantic sediment core GeoB1112 for which other SST proxy data are available. Comparison of the different data sets gives no indication for significant secondary overprinting of the δ44/40Ca signal. Long-term trends in reconstructed SST correlate strongly with temperature records derived from oxygen isotopes and Mg/Ca ratios supporting the methods validity. The observed change of SST of approximately 3 °C at the Holocene-last glacial maximum transition reveals additional evidence for the important role of the tropical Atlantic in triggering global climate change, based on a new independent palaeothermometer.

Description: Chemoherm carbonates, as well as numerous other types of methane seep carbonates, were discovered in 2004 along the passive margin of the northern South China Sea. Lithologically, the carbonates are micritic containing peloids, clasts and clam fragments. Some are highly brecciated with aragonite layers of varying thicknesses lining fractures and voids. Dissolution and replacement is common. Mineralogically, the carbonates are dominated by high magnesium calcites (HMC) and aragonite. Some HMCs with MgCO3 contents of between 30–38 mol%–extreme-HMC, occur in association with minor amounts of dolomite. All of the carbonates are strongly depleted in δ13C, with a range from − 35.7 to − 57.5‰ PDB and enriched in δ18O (+ 4.0 to + 5.3‰ PDB). Abundant microbial rods and filaments were recognized within the carbonate matrix as well as aragonite cements, likely fossils of chemosynthetic microbes involved in carbonate formation. The microbial structures are intimately associated with mineral grains. Some carbonate mineral grains resemble microbes. The isotope characteristics, the fabrics, the microbial structure, and the mineralogies are diagnostic of carbonates derived from anaerobic oxidation of methane mediated by microbes. From the succession of HMCs, extreme-HMC, and dolomite in layered tubular carbonates, combined with the presence of microbial structure and diagenetic fabric, we suggest that extreme-HMC may eventually transform into dolomites. Our results add to the worldwide record of seep carbonates and establish for the first time the exact locations and seafloor morphology where such carbonates formed in the South China Sea. Characteristics of the complex fabric demonstrate how seep carbonates may be used as archives recording multiple fluid regimes, dissolution, and early transformation events.

Description: Calcium isotope fractionation was measured on skeletal aragonite and calcite from different marine biota and on inorganic calcite. Precipitation temperatures ranged from 0 to 28°C. Calcium isotope fractionation shows a temperature dependence in accordance with previous observations: 1000 · ln(αcc) = −1.4 + 0.021 · T (°C) for calcite and 1000 · ln(αar) = −1.9 + 0.017 · T (°C) for aragonite. Within uncertainty the temperature slopes are identical for the two polymorphs. However, at all temperatures calcium isotopes are more fractionated in aragonite than in calcite. The offset in δ44/40Ca is about 0.6‰. The underlying mechanism for this offset may be related to the different coordination numbers and bond strengths of the calcium ions in calcite and aragonite crystals, or to different Ca reaction behavior at the solid-liquid interface. Recently, the observed temperature dependence of the Ca isotope fractionation was explained quantitatively by the temperature control on precipitation rates of calcium carbonates in an experimental setting (Lemarchand et al., 2004). We show that this mechanism can in principle also be applied to CaCO3 precipitation in natural environments in normal marine settings. Following this model, Ca isotope fractionation in marine Ca carbonates is primarily controlled by precipitation rates. On the other hand the larger Ca isotope fractionation of aragonite compared to calcite can not be explained by different precipitation rates. The rate control model of Ca isotope fractionation predicts a strong dependence of the Ca isotopic composition of carbonates on ambient CO32− concentration. While this model is in general accordance with our observations in marine carbonates, cultured specimens of the planktic foraminifer Orbulina universa show no dependence of Ca-isotope fractionation on the ambient CO32− concentration. The latter observation implies that the carbonate chemistry in the calcifying vesicles of the foraminifer is independent from the ambient carbonate ion concentration of the surrounding water.

Description: Neodymium (Nd) isotope profiles were analyzed on two Baltic Mn/Fe precipitates (99/2 and TL1) from shallow water (20 m) of the Mecklenburg Bay. The age range of these Mn/Fe precipitates determined by 226Raex/Ba dating reaches from recent growth back to ∼4300 and 1000 yr BP, respectively. Over this time range, the Nd isotope composition varies from εNd (0) = −13.1 to −17.5 in the selected Baltic precipitates indicating substantial changes in the Nd isotope composition of the Baltic Sea. The lowest εNd values were recorded during the time interval of the Little Ice Age (LIA, AD ∼1350 to 1850). These minimum values indicate either an increase of the input of less radiogenic Nd from Scandinavian Archean-Proterozoic sources (εNd about −22) to the Baltic Sea or a decrease of the input of more radiogenic Nd from continental European sources (εNd about −12) and/or North Sea water (εNd about −10). Variations of both, erosive continental input and North Sea inflow may indicate a direct response of the Nd isotope signal in the Baltic Sea to climate changes during the LIA and be related to cyclic shifts in the atmospheric circulation triggered by the North Atlantic Oscillation (NAO). Another aspect that possibly influenced the input of trace elements and Nd isotopes into the Baltic Sea is the population development in the circum Baltic area during the LIA. The lowest εNd values also correspond to the medieval demographic crises that led to a significant decrease of agricultural activity and farmland. The reduction of soil erosion and enhanced regrowth of natural vegetation may have changed the amount and proportions of dissolved and suspended particulate matter transported into the Baltic Sea by rivers which in turn may have resulted in a change of the Nd isotope composition of Baltic Sea water.

Description: The 230Th content of subsurface Bahamas seawater has been estimated from incrementally deposited aragonite skeletons of sclerosponges using U-series age models which imply syndepositional incorporation of 230Th. First order age models, assuming constant growth rate and zero initial 230Th activity, display an average surface age of 63 ± 18 y despite the fact that the surfaces of the organisms were known to be alive at the time collection in the 1990s. A knowledge of the initial 230Th activity is necessary to incorporate the surface age constraint into an age model. In the Bahamas, this approach estimates an initial 230Th/232Th activity ratio of 4.3 ± 1.1 when applied to several individuals of the Caribbean sclerosponge Ceratoporella nicholsoni. These results differ from the same approach applied to previous work on Jamaican sclerosponges accreting aragonite in an environment closer to crustal sources and with less carbonate substrate. We report new measurements of Jamaican sclerosponge suggesting initial activity ratios of 230Th/232Th of ∼1, albeit with a high amount of uncertainty, indicating a regional difference in initial thorium activities. The estimates of initial 230Th/232Th activity ratios in the Bahamas are similar to the dissolved ratio rather than the particulate ratio and can be explained by mixing of open ocean water and bank-top surface water, however no information regarding temporal changes in the amount of initial 230Th/232Th activity available for incorporation into biogenic aragonite can be constrained using this approach. Growth rates do not change after correction for initial thorium and they display an inverse relationship with depth. Although this paper deals uniquely with sclerosponge aragonite, it is likely that the approach can be used on any incrementally deposited biogenic aragonite, provided information is known a priori about the first order growth model and there is evidence that initial thorium incorporation is representative of ambient environment rather than individual physiology.