Ruth Yam

Isotope composition of air moisture over the Mediterranean Sea: an index of the air–sea interaction pattern

The isotope composition of atmospheric moisture over the Mediterranean Sea, collected during the cruise of the research vessel meteor in January 1995, confirmed that the intensive air—sea interaction near the coast under conditions of a large humidity deficit labels the resultant atmospheric waters with a large deuterium-excess parameter. The present data set shows this effect to result both when cold air from the European continent moves over the sea as well as when warm and dry air from North Africa is involved. The situation in the eastern and western Mediterranean differ in the vertical structure of the isotope composition further away from the coast, as expressed by the gradients of the dexcess values with altitude over the sea surface, i.e. increasing with altitude in the eastern Mediterranean, whereas the opposite effect is noted in the western section and near the coast. A comparison of the isotopic composition of the samples with the expected buildup of moisture over the sea, based on the Craig—Gordon model, suggests that up to one half of the added moisture may have resulted at times from the evaporation of sea-spray droplets, without any significant isotope fractionation, in addition to the vapour-mediated transport from the sea surface, which favors the lighter isotopic species.

Holocene climatic change in Swedish Lapland inferred from an oxygen-isotope record of lacustrine biogenic silica

Holocene climatic variability was studied in a 9500-year lake-sediment sequence from the Abisko region in Swedish Lapland, using the oxygen-isotope ratio in diatom biogenic silica (d18Osi). Oxygen-and hydrogen-isotope ratios of waters from the Abisko area suggest that in this region the evaporative flux is small and the isotopic composition of most lakes reflects that of the local precipitation. The hydrological setting of the region and sensitivity analysis of isotopic response to changing climatic parameters such as humidity, inflow and evaporation show that the downcore diatom d18Osi record is primarily controlled by changes in the summer isotopic composition of the lake water. The overall 3.5‰ depletion in d18Osi since the early Holocene is interpreted as an increase in the influence of the Arctic polar continental air mass that carries depleted precipitation. We estimate that this change is associated with a 2.5–4°C cooling that has occurred since the early Holocene. In general, the diatom d18Osi record resembles the average annual air temperature reconstructed for the Greenland ice core GISP2, especially during the past 4000 years, with a pronounced cooling starting at 2000 years BP.

Stable isotope composition of tropical high-altitude fresh-waters on Mt. Kenya, Equatorial East Africa

Abstract The stable isotope composition of equatorial high-altitude precipitation and surface-water bodies has been studied on Mt. Kenya (0°10′S; 37°20′E) in East Africa during July 1997. Regarding the local hydrology, glacier meltwater was identified as the main source of replenishment for the lakes, while direct precipitation and runoff appear to have a negligible contribution during this period of the year. Two through-flow lakes exhibit nearly no evaporative isotopic enrichment relative to the inflow isotopic composition, while two longer residence-time lakes display an oxygen and hydrogen isotopic build-up of 4.0–7.0‰ and 18.0–34.0‰, respectively. The data indicate that stream waters are composed of a mixture of high-altitude glacier meltwater and rainfall. The isotopic composition of an ice-sample obtained directly from the margin of a glacier is enriched relative to the average composition of the glacier, suggesting mixing with local rain. Samples of ground-frost crystals have an isotopic composition similar to the rain or slightly enriched. In addition, the results show the lack of a significant “altitude effect” on the mountain. This study demonstrates the importance of the characterization of local hydrological settings when interpreting lacustrine equatorial isotopic records of past climate and confirms that for East Africa, the total range in oxygen and hydrogen isotopic composition of meteoric waters is smaller than in high-latitudes.

Recent unprecedented warming and oligotrophy of the eastern Mediterranean Sea within the last millennium

The Mediterranean region is a climatic transitional zone between the subtropical/monsoon regime and the temperate westerlies and is subject to forces acting upon the global climate system. Much knowledge about its climate over the last millennium is derived from terrestrial records, whereas changes in sea surface temperatures (SSTs) and in the dissolved inorganic carbon pool (DIC) are poorly known. We present continuous high-resolution reconstructions of SST and δ13CDIC in the eastern Mediterranean (EM) Sea, as inferred from oxygen and carbon isotope records from the skeletons of the reef builder gastropod Dendropoma sp. Spanning the past millennium, the SST reconstruction reveals a 250 year persistent warming trend during which the twentieth century was the warmest on record. Coupled with a distinct trend of 13CDIC depletion and superimposed upon decreased primary production, this climate reconstruction reflects a new state of the EM over the Anthropocene era that exceeds the natural variability of the last millennium.

Sea ice diatom contributions to Holocene nutrient utilization in East Antarctica

Combined high-resolution Holocene δ30Sidiat and δ13Cdiat paleorecords are presented from the Seasonal Ice Zone, East Antarctica. Both data sets reflect periods of increased nutrient utilization by diatoms during the Hypsithermal period (circa 7800 to 3500 calendar years (cal years) B.P.), coincident with a higher abundance of open water diatom species (Fragilariopsis kerguelensis), increased biogenic silica productivity (%BSi), and higher regional summer temperatures. The Neoglacial period (after circa 3500 cal years B.P.) is reflected by an increase in sea ice indicative species (Fragilariopsis curta and Fragilariopsis cylindrus, up to 50%) along with a decrease in %BSi and δ13Cdiat (< −18‰ to −23‰). However, over this period, δ30Sidiat data show an increasing trend, to some of the highest values in the Holocene record (average of +0.43‰). Competing hypotheses are discussed to account for the decoupling trend in utilization proxies including iron fertilization, species-dependent fractionation effects, and diatom habitats. Based on mass balance calculations, we highlight that diatom species derived from the semi-enclosed sea ice environment may have a confounding effect upon δ30Sidowncore compositions of the seasonal sea ice zone. A diatom composition, with approximately 28% of biogenic silica derived from the sea ice environment (diat-SI) can account for the increased average composition of δ30Sidiat during the Neoglacial. These data highlight the significant role sea ice diatoms can play with relation to their export in sediment records, which has implications on productivity reconstructions from the seasonal ice zone.

Basin-scale estimates of pelagic and coral reef calcification in the Red Sea and Western Indian Ocean

Significance This approach of estimating basin-scale calcification rates and the relative role of margins (reefs) versus pelagic carbonate deposition can be applied as a long-term monitoring scheme for the effects of ocean acidification. The database for the response of individual organisms to elevated CO2 levels is growing steadily, but in situ measurements assessing this response are very local. The study demonstrates the feasibility of estimating the relative calcification rates of planktonic and benthic populations over a whole basin and provides a tool for the monitoring of these complex systems on large spatial scales. Basin-scale calcification rates are highly important in assessments of the global oceanic carbon cycle. Traditionally, such estimates were based on rates of sedimentation measured with sediment traps or in deep sea cores. Here we estimated CaCO3 precipitation rates in the surface water of the Red Sea from total alkalinity depletion along their axial flow using the water flux in the straits of Bab el Mandeb. The relative contribution of coral reefs and open sea plankton were calculated by fitting a Rayleigh distillation model to the increase in the strontium to calcium ratio. We estimate the net amount of CaCO3 precipitated in the Red Sea to be 7.3 ± 0.4·1010 kg·y−1 of which 80 ± 5% is by pelagic calcareous plankton and 20 ± 5% is by the flourishing coastal coral reefs. This estimate for pelagic calcification rate is up to 40% higher than published sedimentary CaCO3 accumulation rates for the region. The calcification rate of the Gulf of Aden was estimated by the Rayleigh model to be ∼1/2 of the Red Sea, and in the northwestern Indian Ocean, it was smaller than our detection limit. The results of this study suggest that variations of major ions on a basin scale may potentially help in assessing long-term effects of ocean acidification on carbonate deposition by marine organisms.

Late Holocene climatic change in the Balkans: Speleothem isotopic data from Serbia

A detailed profile of the stable isotopes of carbon and oxygen was obtained from a speleothem (stalagmite) from the Ceremosjna Cave in eastern Serbia. The stalagmite is a low magnesian calcite that did not show any evidence of diagenetic alteration. It was precipitated under isotopic equilibrium conditions from dripping water. The age and rate of deposition was derived from six internally consistent radiocarbon dates. The initial (super 14) C activity was determined to be approximately 80 pMC. The stalagmite appears to preserve a continuous record of calcite deposition from approximately 2300 BP until the present. Oxygen isotopic data, based upon 100 samples, are used to derive the first paleotemperature record for Serbia. A regression analysis of the all the data indicates that over the period of time that the speleothem was deposited there was a general trend of lowering of the average temperature. Superimposed upon this are significant long-term temperature fluctuations. These can be divided into four broader climatic groupings. Going from the oldest times to the present, there are two warm periods separated by a period when the temperatures fell below the temperature trend line. However, the absolute temperatures were generally above those of the more recent period that is generally characterized by the coolest climatic conditions.

Evidence for Rhythmicity Pacemaker in the Calcification Process of Scleractinian Coral

Reef-building scleractinian (stony) corals are among the most efficient bio-mineralizing organisms in nature. The calcification rate of scleractinian corals oscillates under ambient light conditions, with a cyclic, diurnal pattern. A fundamental question is whether this cyclic pattern is controlled by exogenous signals or by an endogenous ‘biological-clock’ mechanism, or both. To address this problem, we have studied calcification patterns of the Red Sea scleractinian coral Acropora eurystoma with frequent measurements of total alkalinity (AT) under different light conditions. Additionally, skeletal extension and ultra-structure of newly deposited calcium carbonate were elucidated with 86Sr isotope labeling analysis, combined with NanoSIMS ion microprobe and scanning electron microscope imaging. Our results show that the calcification process persists with its cyclic pattern under constant light conditions while dissolution takes place within one day of constant dark conditions, indicating that an intrinsic, light-entrained mechanism may be involved in controlling the calcification process in photosymbiotic corals.

Identifying genes and regulatory pathways associated with the scleractinian coral calcification process

Reef building corals precipitate calcium carbonate as an exo-skeleton and provide substratum for prosperous marine life. Biomineralization of the coral’s skeleton is a developmental process that occurs concurrently with other proliferation processes that control the animal extension and growth. The development of the animal body is regulated by large gene regulatory networks, which control the expression of gene sets that progressively generate developmental patterns in the animal body. In this study we have explored the gene expression profile and signaling pathways followed by the calcification process of a basal metazoan, the Red Sea scleractinian (stony) coral, Stylophora pistillata. When treated by seawater with high calcium concentrations (addition of 100 gm/L, added as CaCl2.2H2O), the coral increases its calcification rates and associated genes were up-regulated as a result, which were then identified. Gene expression was compared between corals treated with elevated and normal calcium concentrations. Calcification rate measurements and gene expression analysis by microarray RNA transcriptional profiling at two time-points (midday and night-time) revealed several genes common within mammalian gene regulatory networks. This study indicates that core genes of the Wnt and TGF-β/BMP signaling pathways may also play roles in development, growth, and biomineralization in early-diverging organisms such as corals.