Ulrich Struck

Temperature and salinity variations of Mediterranean Sea surface waters over the last 16,000 years from records of planktonic stable oxygen isotopes and alkenone unsaturation ratios

Alkenone unsaturation ratios and planktonic d18O records from sediment cores of the Alboran, Ionian and Levantine basins in the Mediterranean Sea show pronounced variations in paleo-temperatures and -salinities of surface waters over the last 16,000 years. Average sea surface temperatures (SSTs) are low during the last glacial (averages prior to 13,000 years: 11‐15°C ), vary rapidly at the beginning of the Holocene, and increase to 17‐18° Ca t all sites during S1 formation (dated between 9500 and 6600 calendar years). The modern temperature gradient (2‐ 3°C ) between the Mediterranean sub-basins is maintained during formation of sapropel S1 in the Eastern Mediterranean Sea. After S1, SSTs have remained uniform in the Alboran Sea at 18°C and have fluctuated around 20°C in the Ionian and Levantine Basin sites. The d18O of planktonic foraminifer calcite decreases by 2‰ from the late glacial to S1 sediments in the Ionian Basin and by 2.8‰ in the Levantine Basin. In the Alboran Sea, the decrease is 1.7‰. Of the 2.8‰ decrease in the Levantine Basin, the eVect of global ice volume accounts for a maximum of 1.05‰ and the temperature increase explains only a maximum of 1.3‰. The remainder is attributed to salinity changes. We use the temperature and salinity estimates to calculate seawater density changes. They indicate that a reversal of water mass circulation is not a likely explanation for increased carbon burial during S1 time. Instead, it appears that intermediate and deep water formation may have shifted to the Ionian Sea approximately 2000 years before onset of S1 deposition, because surface waters were as cold, but saltier than surface water in the Levantine Basin during the Younger Dryas. Sapropel S1 began to form at the same time, when a significant density decrease also occurred in the Ionian Sea.

A multiproxy reconstruction of the evolution of deep and surface waters in the subarctic Nordic seas over the last 30,000yr

On the basis of various lithological, mircopaleontological and isotopic proxy records covering the last 30,000 calendar years (cal kyr) the paleoenvironmental evolution of the deep and surface water circulation in the subarctic Nordic seas was reconstructed for a climate interval characterized by intensive ice-sheet growth and subsequent decay on the surrounding land masses. The data reveal considerable temporal changes in the type of thermohaline circulation. Open-water convection prevailed in the early record, providing moisture for the Fennoscandian-Barents ice sheets to grow until they reached the shelf break at ∼26 cal. kyr and started to deliver high amounts of ice-rafted debris (IRD) into the ocean via melting icebergs. Low epibenthic δ18O values and small-sized subpolar foraminifera observed after 26 cal. kyr may implicate that advection of Atlantic water into the Nordic seas occurred at the subsurface until 15 cal. kyr. Although modern-like surface and deep-water conditions first developed at ∼13.5 cal. kyr, thermohaline circulation remained unstable, switching between a subsurface and surface advection of Atlantic water until 10 cal. kyr when IRD deposition and major input of meltwater ceased. During this time, two depletions in epibenthic δ13C are recognized just before and after the Younger Dryas indicating a notable reduction in convectional processes. Despite an intermittent cooling at ∼8 cal. kyr, warmest surface conditions existed in the central Nordic seas between 10 and 6 cal. kyr. However, already after 7 cal. kyr the present day situation gradually evolved, verified by a strong water mass exchange with the Arctic Ocean and an intensifying deep convection as well as surface temperature decrease in the central Nordic seas. This process led to the development of the modern distribution of water masses and associated oceanographic fronts after 5 cal. kyr and, eventually, to todays steep east–west surface temperature gradient. The time discrepancy between intensive vertical convection after 5 cal. kyr but warmest surface temperatures already between 10 and 6 cal. kyr strongly implicates that widespread postglacial surface warming in the Nordic seas was not directly linked to the rates in deep-water formation.

Eastern Mediterranean surface water temperatures and d18O composition during deposition of sapropels in the late Quaternary

0indices) and d 18 O of planktonic foraminifer calcite (d 18 Ofc) across late Pleistocene sapropel intervals show that d 18 Ofc decreased (between 1 and 4.6%) and SST increased (between 0.7� and 6.7� C). Maximal d 18 Oseawater depletion of eastern Mediterranean surface waters at the transition is between 0.5 and 3.0%, and in all but one case exceeded the depletion seen in a western Mediterranean core. The depletion in d 18 Oseawater is most pronounced at sapropel bases, in agreement with an initial sudden input of monsoon-derived freshwater. Most sapropels coincide with warming trends of SST. The density decrease by initial freshwater input and continued warming of the sea surface pooled fresh water in the surface layer and prohibited deep convection down to ageing deep water emplaced during cold and arid glacial conditions. An exception to this pattern is ‘‘glacial’’ sapropel S6; its largest d 18 Oseawater depletion (3%) is almost matched by the depletion in the western Mediterranean Sea, and it is accompanied by surface water cooling following an initially rapid warming phase. A second period of significant isotopic depletion is in isotope stage 6 at the 150 kyr insolation maximum. While not expressed as a sapropel due to cold SST, it is in accord with a strengthened monsoon in the southern catchment. INDEX TERMS: 1055 Geochemistry: Organic geochemistry; 1620 Global Change: Climate dynamics (3309); 4267 Oceanography: General: Paleoceanography; 9604 Information Related to Geologic Time: Cenozoic; KEYWORDS: Mediterranean Sea, sapropels, sea surface temperatures, oxygen isotopes, Quaternary Citation: Emeis, K.-C., et al., Eastern Mediterranean surface water temperatures and d 18 O composition during deposition of sapropels

Stable nitrogen and carbon isotopes as indicator of eutrophication of the Oder river (Baltic sea)

Abstract For the Pomeranian Bight of the Baltic Sea the influence of riverine POM is clearly reflected in the isotopic ( δ 15 N , δ 13 C ) signature of suspended matter and surface sediments. High δ 15 N values of 10–14%. characterize recent river borne particulates and surface sediments along the outflow routes and in the upper 10–15 cm of sediment cores from the lagoon. Surface sediments of the Arkona Basin, 100 nautical miles from the river mouth, have δ 15 N values of 6%. which is assumed to be a mixture of autochthonous and riverine POM. The high δ 15 N is likely due to isotopic fractionation during partial utilization of DIN in the lagoon. Carbon isotopes show a similar spatial and temporal pattern with a value of − 28.3%. in surface sediments of the lagoon and a rapid increase along the western outflow route towards the Arkona Basin with − 23.8%. in the surface sediments, which is characteristic for marine organic matter. Delta 13 C in the lagoon sediments decrease downcore to lower than − 29%., which is typical for terrigenous matter. The Arkona Basin sediments, however, show a slight increase in δ 13 C with depth. The higher modern δ 13 C in the lagoon is likely due to CO 2 limitation resulting from eutrophication and high primary production in river and lagoon. Changes in both C and N isotopes of sediments from cores dated using 210 Pb indicate an increasing nutrient load of the river over the last 50 years. Using calculated mixing rates an input from the Oder river of 60 to 95% of nitrogenous material and 50–60% POC into the Pomeranian Bight and 34% PON and 13% POC into the Arkona Basin is calculated for today.

Biological productivity during sapropel S5 formation in the Eastern Mediterranean Sea: Evidence from stable isotopes of nitrogen and carbon

We determined 15N/14N ratios in modern surface and sapropel S5 sediments of the Mediterranean Sea to clarify differences in the nutrient regime associated with sapropel formation. In the modern situation, high δ15N of unused nitrate (15–20 ‰) remaining in the surface waters during the winter phytoplankton bloom evidences P-limitation of biological production in winter. δ15N of surface sediments decrease towards the east of the basin (5 to >2.5‰). This is a consequence of either eastward increasing nitrogen fixation during the summer months, or of particulate matter being supplied predominantly by the P-limited winter bloom. Very low (−1–1‰) δ15N values in sapropel S-5 from four locations require a very light source of nutrient-N assimilated at a minimum of ten times the modern export flux. Because the isochronous records show no spatial gradient in δ15N, we exclude both Ekman-type upwelling and direct riverine discharge as likely sources of nutrients. Our data are consistent with an anti-estuarine thermohaline circulation in the upper 500m during S5 time, allowing for the trapping of nutrients in the eastern basin. The most likely scenario for S5 is that phosphorus release from a relatively shallow redox boundary resulted in an imbalanced supply of N:P (<16:1) to the photic zone. The result was a slow assimilation of carbon during summer stratification and extensive N2-fixation providing the majority of the export flux from a N-limited system.

An early Hettangian coral reef in southern France: Implications for the end-Triassic reef crisis

Abstract The oldest known Jurassic coral reef is exposed in the Ardèche region of southern France. This reef site, consisting of at least three reefal bodies, is of early Hettangian age and thus immediately postdates the end-Triassic mass extinction, which is well known for its catastrophic effect on reef building. Bulk carbonate carbon isotopes of the limestones below the reef are likely to record environmental perturbations subsequent to the mass extinction. The main reef is surprisingly well developed (20 m in thickness, 200 m in lateral extent) and composed of at least four genera and six species of corals—not only holdover genera from the Triassic, but also one newly evolved genus (Phacelophyllia), contributed to reef construction. Just like their latest Triassic counterparts, the reef is dominated by phaceloid corals with a considerable contribution of microbialite. The reef predates similarly well developed structures by almost ten million years. The shelf setting of the reef renders it unlikely that refuges around oceanic islands are needed to explain survival of corals across the end-Triassic mass extinction.

Coral mucus fuels the sponge loop in warm- and cold-water coral reef ecosystems

Shallow warm-water and deep-sea cold-water corals engineer the coral reef framework and fertilize reef communities by releasing coral mucus, a source of reef dissolved organic matter (DOM). By transforming DOM into particulate detritus, sponges play a key role in transferring the energy and nutrients in DOM to higher trophic levels on Caribbean reefs via the so-called sponge loop. Coral mucus may be a major DOM source for the sponge loop, but mucus uptake by sponges has not been demonstrated. Here we used laboratory stable isotope tracer experiments to show the transfer of coral mucus into the bulk tissue and phospholipid fatty acids of the warm-water sponge Mycale fistulifera and cold-water sponge Hymedesmia coriacea, demonstrating a direct trophic link between corals and reef sponges. Furthermore, 21–40% of the mucus carbon and 32–39% of the nitrogen assimilated by the sponges was subsequently released as detritus, confirming a sponge loop on Red Sea warm-water and north Atlantic cold-water coral reefs. The presence of a sponge loop in two vastly different reef environments suggests it is a ubiquitous feature of reef ecosystems contributing to the high biogeochemical cycling that may enable coral reefs to thrive in nutrient-limited (warm-water) and energy-limited (cold-water) environments.

Tracking seasonal changes in North Sea zooplankton trophic dynamics using stable isotopes

Trophodynamics of meso-zooplankton in the North Sea (NS) were assessed at a site in the southern NS, and at a shallow and a deep site in the central NS. Offshore and neritic species from different ecological niches, including Calanus spp., Temora spp. and Sagitta spp., were collected during seven cruises over 14 months from 2007 to 2008. Bulk stable isotope (SI) analysis, phospholipid-derived fatty acid (PLFA) compositions, and δ13CPLFA data of meso-zooplankton and particulate organic matter (POM) were used to describe changes in zooplankton relative trophic positions (RTPs) and trophodynamics. The aim of the study was to test the hypothesis that the RTPs of zooplankton in the North Sea vary spatially and seasonally, in response to hydrographic variability, with the microbial food web playing an important role at times. Zooplankton RTPs tended to be higher during winter and lower during the phytoplankton bloom in spring. RTPs were highest for predators such as Sagitta sp. and Calanus helgolandicus and lowest for small copepods such as Pseudocalanus elongatus and zoea larvae (Brachyura). δ15NPOM-based RTPs were only moderate surrogates for animals’ ecological niches, because of the plasticity in source materials from the herbivorous and the microbial loop food web. Common (16:0) and essential (eicosapentaenoic acid, EPA and docosahexaenoic acid, DHA) structural lipids showed relatively constant abundances. This could be explained by incorporation of PLFAs with δ13C signatures which followed seasonal changes in bulk δ13CPOM and PLFA δ13CPOM signatures. This study highlighted the complementarity of three biogeochemical approaches for trophodynamic studies and substantiated conceptual views of size-based food web analysis, in which small individuals of large species may be functionally equivalent to large individuals of small species. Seasonal and spatial variability was also important in altering the relative importance of the herbivorous and microbial food webs.

Tracing the diet of the monitor lizard Varanus mabitang by stable isotope analyses (δ15N, δ13C)

Abstract. In this study, we used analyses of stable isotopes of nitrogen (δ15N) and carbon (δ13C) to determine the trophic ecology of the monitor lizard Varanus mabitang. Stable isotopes from claws, gut contents, and soft tissues were measured from the type specimen. Samples from Varanus olivaceus, Varanus prasinus, Varanus salvator, the herbivorous agamid lizard Hydrosaurus pustulatus, and some plant matter were included for comparison. Our data show a rapid decrease in δ13C (about10‰) from food plants towards gut contents and soft tissues of herbivorous species. For the varanids, we found a significant linear correlation of decreasing δ13C and increasing δ15N from herbivorous towards carnivorous species. In terms of trophic isotope ecology, the type specimen of V. mabitang is a strict herbivore. Thus it differs significantly in its isotopic composition from the morphologically next closest related species V. olivaceus. The most highly carnivorous species is V. salvator, while δ15N values for V. prasinus and V. olivaceus are intermediate. Claws provide very valuable samples for such measurements, because they can be sampled from living animals without harm. Additionally, their range of variability is relatively small in comparison with measurements from soft tissues.

Phosphoric acid fractionation factors for smithsonite and cerussite between 25 and 72°C

The intramolecular kinetic oxygen isotope fractionation between CO2 and CO32− during reaction of phosphoric acid with natural smithsonite (ZnCO3) and cerussite (PbCO3) has been determined between 25 and 72°C. While cerussite decomposes in phosphoric acid within a few hours at 25°C, smithsonite reacts very slowly with the acid at 25°C providing yields of CO2 < 25% after 2 weeks. The low yields result in a low precision for oxygen isotope measurements of the acid-liberated CO2 (±1.65‰, 1σ, n = 9). The yield and reproducibility of oxygen isotope values of the acid-liberated CO2 from smithsonite can be improved, the latter to ∼±0.15‰, by increasing the reaction temperature to 50°C for 12 h or to 72°C for 1 h. Our new phosphoric acid fractionation factor for natural cerussite at 25°C deviates significantly from a previously published value on synthetic material. The temperature dependence of the oxygen isotope factionation factor, α between acid-liberated CO2 and carbonate at 25 to 72°C is given by the following equations with temperature T in kelvin. A comparison with published equations of the form 1000lnαphosCO2-carbonate = A + B × 105/T2 for other divalent metal carbonates shows that the factors B of slowly-reacting carbonates from the rhombohedral calcite group (magnesite, siderite, smithsonite and rhodochrosite) are very similar (6.7 ± 0.2) and distinct from those of fast-reacting minerals witherite, cerussite, and strontianite of the orthorhombic aragonite group (4.5 ± 0.3) and calcite (5.6 ± 0.1). These differences indicate a crystallographic control on the temperature dependence of the kinetic oxygen isotope fractionation between phosphoric acid liberated CO2 and carbonate.