Aldo Shemesh

Silicon-isotope composition of diatoms as an indicator of past oceanic change

Silicon is essential for the growth of diatoms, a group of phytoplankton with opal (amorphous hydrated silica) shells. Diatoms largely control the cycling of silicon in the ocean and, conversely, diatom silica production rates can be limited by the availability of silicic acid. Diatoms are biogeochemically important in that they account for an estimated 75% of the primary production occurring in coastal and nutrient-replete waters, rising to more than 90% during ice-edge blooms such as occur in the Ross Sea, off Antarctica. There are few means by which to reconstruct the history of diatom productivity and marine silicon cycling, and thus to explore the potential contribution of diatoms to past oceanic biogeochemistry or climate. Indices based on the accumulation of sedimentary opal are often biased by the winnowing and focusing of sediments and by opal dissolution. Normalization of opal accumulation records using particle-reactive natural radionuclides may correct for sediment redistribution artefacts and the dissolution of opal within sediments,, but not for opal dissolution before it arrives at the sea floor. Half of the opal produced in the euphotic zone may dissolve before sinking to a depth of 200 m (ref. 1), constituting a potentially large bias to both normalized and uncorrected records of opal accumulation. Here we exploit the potential that variations in the ratio of 30Si to 28Si in sedimentary opal may provide information on past silicon cycling that is unbiased by opal dissolution. Our silicon stable-isotope measurements suggest that the percentage utilization of silicic acid by diatoms in the Southern Ocean during the last glacial period was strongly diminished relative to the present interglacial.

Oxygen isotope variations in phosphate of biogenic apatites, II. Phosphorite rocks

Abstract Phosphorites from sedimentary sequences ranging in age from Archaean to Recent were analysed for δ 18 O in both the PO 4 (δ 18 O p ) and CO 3 (δ 18 O c ) in the apatite lattice. The oxygen isotope record is considerably better preserved in phosphates than in either carbonates or cherts. The use of the Longinelli and Nuti [8] temperature equation yields temperatures for Recent phosphorites that are in good agreement with those measured in the field. The δ 18 O p values of ancient phosphorites decrease with increasing age. These changes with time are not likely to be due to post-depositional exchange. Changes in δ 18 O values of seawater and variations of temperature with time can account for the δ 18 O p time trend, but the latter explanation is preferred. In Ancient phosphorites δ 18 O c in structurally bound carbonate in apatite is not a reliable geochemical indicator.

Crystallinity and diagenesis of sedimentary apatites

The crystallinity of sedimentary apatites was determined by Fourier transform infrared spectroscopy (FT-IR) using the splitting of a triply degenerate antisymmetric bending vibration of orthophosphate. The crystallinity indices of Recent marine apatites are low (3.0–3.6) while those of onland ancient apatites are high (4.5–7.8), indicating post-depositional recrystallization. The infrared spectra reveal that recrystallization is associated with a decrease in carbonate content substituting for PO43− and an increase in fluoride order within the apatite structure. The relationship between the crystallinity index and PO43− δ 18O suggests alteration of the primary isotopic composition by exchange reactions between PO43− oxygens and surrounding waters. The Monterey samples have a large range of crystallinity index that reflects a set of complex and highly variable diagenetic conditions. This demonstrates the use of FT-IR criteria for differentiating between pristine and altered apatites and, as a consequence, for relating geochemical markers to formation or diagenetic environments. It is suggested that only those samples that have low crystallinity indices (C. I. < 3.8) should be considered as pristine apatite. Spectra of fish remains indicate that differences in rare earth element (REE) patterns correspond to variations in crystallinity, carbonate content and F order in the apatite lattice. The fact that crystallinity is not correlated with geologic age suggests that environmental factors, such as accumulation rate and pore water chemistry, govern the recrystallization process. In general, Sr content decreases and δ 18Op exhibits high variability with increasing crystallinity.

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.

Oxygen Isotopes in Biogenic Silica: Global Changes in Ocean Temperature and Isotopic Composition

A record of oxygen isotopes in biogenic silica from a deep-sea sediment core from the Southern Ocean reveals that marine diatoms retain their primary isotopic composition after burial. As a result, the marine diatom record can be combined with data on coexisting planktonic foraminifera to monitor past surface temperature and isotopic composition of seawater. The coupling of these two records allows the solution of two paleotemperature equations for each core interval. Data from a South Atlantic core show that the average δ18O during the glacial period at this site was higher by about 1.3 per mil than average Holocene values, and that average glacial-age temperatures were not significantly different from average Holocene values.

Isotopic evidence for reduced productivity in the glacial Southern Ocean

Records of carbon and nitrogen isotopes in biogenic silica and carbon isotopes in planktonic foraminifera from deep-sea sediment cores from the Southern Ocean reveal that the primary production during the last glacial maximum was lower than Holocene productivity. These observations conflict with the hypothesis that the low atmospheric carbon dioxide concentrations were introduced by an increase in the efficiency of the high-latitude biological pump. Instead, different oceanic sectors may have had high glacial productivity, or alternative mechanisms that do not involve the biological pump must be considered as the primary cause of the low glacial atmospheric carbon dioxide concentrations.

Sequence of events during the last deglaciation in Southern Ocean sediments and Antarctic ice cores

variation depends on the age model as 14 C determinations cannot be obtained for the time interval of 29.5–14.5 ka. Assuming a constant sedimentation rate for this interval, our data suggest that sea ice and nutrient changes at about 19 ka B.P. lead the increase in atmospheric pCO2 by approximately 2000 years. Our diatom-based sea ice record is in phase with the sodium record of the Vostok ice core, which is related to sea ice cover and similarly leads the increase in atmospheric CO2. If gas exchange played a major role in determining glacial to interglacial CO2 variations, then a delay mechanism of a few thousand years is needed to explain the observed sequence of events. Otherwise, the main cause of atmospheric pCO2 change must be sought elsewhere, rather than in the Southern Ocean. INDEX TERMS: 3344 Meteorology and Atmospheric Dynamics: Paleoclimatology; 4267 Oceanography: General: Paleoceanography; 4845 Oceanography: Biological and Chemical: Nutrients and nutrient cycling; 4870 Oceanography: Biological and Chemical: Stable isotopes; 9325 Information Related to Geographic Region: Atlantic Ocean; KEYWORDS: biogenic opal, stable isotopes, diatoms, IRD, last glacial, Southern Ocean

Late Pleistocene oxygen isotope records of biogenic silica from the Atlantic sector of the Southern Ocean

We determined the isotopic composition of oxygen in marine diatoms in eight deep-sea cores recovered from the Atlantic sector of the Southern Ocean. The analytical reproducibility and core-to-core consistency of the isotopic signal suggests that diatom δ18O can be used as a new paleocenographic tool to reconstruct past variations in surface water characteristics and to generate 18O -isotope-based stratigraphy for the Southern Ocean. The data indicate that diatom δ18O reflects sea surface temperature and seawater isotopic composition and that diatoms retain their isotopic signal on timescales of a least 430 ka. The δ18O analyses of different diatom assemblages reveal that the isotopic signal is free of species effects and that the common Antarctic species have the same water-opal fractionation. The transition from the last glacial maximum (LGM) to the Holocene is fully recorded in high sedimentation rate cores. An 18O enrichment during the LGM, a post-LGM meltwater spike and an input of meltwater during the late Holocene are the main isotopic features observed in down core records. The origin of this meltwater was very likely melting icebergs and/or continental ice or by melting sea ice that had accumulated snow. The most pronounced meltwater effects are recorded in cores that are associated with the Weddel gyre. Our results provide the basis for extending isotope studies to oceanic regions devoid of carbonate; further, isotopic stratigraphies may be constructed for records and regions where they were previously not possible.

High-resolution record of climate stability in France during the last interglacial period

The last interglacial period (127–110 kyr ago) has been considered to be an analogue to the present interglacial period, the Holocene, which may help us to understand present climate evolution. But whereas Holocene climate has been essentially stable in Europe, variability in climate during the last interglacial period has remained unresolved, because climate reconstructions from ice cores, continental records and marine sediment cores give conflicting results for this period. Here we present a high-resolution multi-proxy lacustrine record of climate change during the last interglacial period, based on oxygen isotopes in diatom silica, diatom assemblages and pollen–climate transfer functions from the Ribains maar in France. Contrary to a previous study, our data do not show a cold event interrupting the warm interglacial climate. Instead, we find an early temperature maximum with a transition to a colder climate about halfway through the sequence. The end of the interglacial period is clearly marked by an abrupt change in all proxy records. Our study confirms that in southwestern Europe the last interglacial period was a time of climatic stability and is therefore still likely to represent a useful analogue for the present climate.

The Stable Isotope Composition of Waters of the Eastern Mediterranean Sea

Waters of the eastern Mediterranean portray an unusual pattern of stable isotope composition compared with other evaporitic systems: an increase in 18O concentration up to value of δ18O = +2.2‰ is not matched by a commensurate increase of deuterium. It is shown that this unusual pattern is an expression of the “medi-terranean” location of the sea, where the air-sea interaction with relatively dry and isotopically depleted continental air masses dominates the evaporation process in winter and where the diluting meteoric waters are extremely depleted in the heavy isotopes. As a result, the slope of the evaporation line in δ(2H)-δ(18O) space is lower than in other marine systems, whereas the mixing line between the meteoric waters and the seawater is very steep. This pattern provides an independent method for the estimation of the evaporation E and freshwater influx terms M of the water balance of the Mediterranean Sea. In winter, the ratio of evaporation to freshwater input is found to be E/M = 1.20, whereas in summer this ratio E/M = 1.83.