Ostracod Valves Research Articles

Amino acid racemization in lacustrine ostracodes, part II: Paleothermometry in Pleistocene sediments at Summer Lake, Oregon

The extent of amino acid racemization in fossil ostracodes of known age has served as a valuable proxy indicator of Pleistocene temperatures in the Great Basin of the western U.S. Here we assess the utility of this technique for estimating paleotemperatures in the Summer Lake basin of south-central Oregon. The enantiomeric composition ( D/ L values) of aspartic acid and glutamic acid was measured in Candona and Limnocythere ostracode valves collected from 32 sedimentary horizons within a ∼16-m section of lacustrine sediments associated with pluvial Lake Chewaucan and exposed along the Ana River. Independent geochronologic control spanning the past 250,000 years is provided by a recently revised age-depth model for the Ana River section. Measured D/ L values in the fossil ostracodes generally increase down-section, but the trend is not monotonic and contains stratigraphic reversals. The D/ L values from the Ana River section are significantly lower than those measured in ostracodes of comparable age from Lake Bonneville sediments, implying that the Summer Lake basin was colder than the Bonneville basin during the middle and late Quaternary. Although a higher north-south temperature gradient during the Pleistocene is expected considering the proximity of the Ana River site to former ice sheets, published amino acid paleothermometry equations for Candona yield unreasonably cold Pleistocene temperatures at Summer Lake. Measurements of fossil Candona valves from deposits across the western U.S. and northern Sonora reveal abnormally high amino acid concentrations in the subset of Ana River ostracodes analyzed, possibly related to unusual preservation and environmental factors at this site. This suggests that ostracode preservation has an influence on rates of racemization which complicates the application of amino acid paleothermometry at the Ana River site.

Amino acid racemization in lacustrine ostracodes, part I: effect of oxidizing pre-treatments on amino acid composition

Bleaching biominerals reduces the concentration of amino acids to a residual fraction which is thought to be preserved within intra-crystalline proteins. Intra-crystalline amino acids represent a relatively closed-system and may be superior for amino acid geochronological and paleothermometry investigations. Here we quantify the effects of extended bleaching (to 144 h) on amino acid concentrations and d/ l values in the valves of seven modern ostracode species. Then we compare the effects of increasingly aggressive pre-treatments (sonication, exposure to 3% hydrogen peroxide for 2 h, and exposure to 10% bleach for 48 h) on amino acid concentrations and d/ l values in fossil ostracode ( Candona) valves from numerous locations throughout the western United States and northern Mexico. Bleaching modern ostracode valves results in a rapid order-of-magnitude reduction in amino acid concentrations and a slight increase in d/ l values. Amino acid concentrations in five of the seven modern species increased slightly after prolonged exposure to bleach, suggesting that bleaching weakens a resistant protein source within the ostracode valves. Increasingly aggressive pre-treatments had virtually no effect on amino acid concentrations, d/ l values, or inter-sample variability in fossil Candona valves older than about 1 ka. Candona valves younger than about 1 ka showed a progressive decrease in amino acid concentrations with more aggressive oxidizing pre-treatments. Valves older than 1 ka generally had amino acid concentrations < 10 nM mg −1. A delayed increase in amino acid concentrations (15–30 nM mg −1), followed by a gradual decrease to residual concentrations consistently < 10 nM mg −1, was observed in both outcrop and core samples. We propose that a large amount of weakly bound proteins is leached geologically instantaneously from Candona valves. Long-term (10’s of ka) diagenesis gradually weakens and then destroys a secondary pool of tightly bound proteins contained within the valves. Residual amino acid concentrations are likely derived from intra-crystalline proteins and resistant inter-crystalline proteins that are essentially immune to leaching. Candona valves deposited in pluvial Lake Chewaucan and exposed in sediments that crop out along the Ana River, Oregon, have unexpectedly well-preserved proteins and lower d/ l values compared to fossil Candona valves of similar age from other locations. The Ana River results argue for developing site-specific calibrations for amino acid geochronology or paleothermometry studies that utilize ostracodes.

Ostracod-based isotope record from Lake Ohrid (Balkan Peninsula) over the last 140 ka

The stable isotope composition of benthic ostracods from a deep-lake sediment core (JO2004-1) recovered from Lake Ohrid (Albania-Macedonia) was studied to investigate regional responses to climate change at the interface between the north-central European and Mediterranean climate systems. Ostracod valves are present only during interglacial intervals, during the Marine Isotope Stage (MIS) 5 and 1. The ostracod oxygen isotope values (δ 18O) quantitatively reflect changes in the oxygen isotope signal of the lake water (δ 18O L). The interpretation of this record however, is far from straight forward. δ 18O L variations throughout MIS 5/6 transition (TII), MIS 5 and MIS 1 appear to be controlled by site specific hydrological processes as shown by modern isotope hydrology. The δ 18O L trends at TII, MIS 5 and MIS 1 match the timing and the main structural feature of the major regional climate records (Corchia cave δ 18O, Iberian margin Sea Surface Temperature) suggesting that the Ohrid δ 18O L responded to global-scale climate changes, although it seems certain that the lake experienced a significant degree of evaporation and varying moisture availability. The carbon isotope signal (δ 13C) seems to respond more accurately to climate changes in agreement with other JO2004-1 proxies. δ 13C of the ostracod calcite is directly linked to the δ 13C of the dissolved inorganic carbon (DIC) in the lake, which in this case is controlled by the isotopic composition of the DIC in the incoming water and by the internal processes of the lake. High δ 13C during cold periods and low values during warm periods reflect changing vegetation cover and soil activity. These results suggest that Lake Ohrid has the potential to capture a long record of regional environment related-temperature trends during interglacial periods, particularly given the exceptional thickness of the lake sediment covering probably the entire Quaternary.

Episodes of low dissolved oxygen indicated by ostracodes and sediment geochemistry at Crystal Lake, Illinois, USA

Low dissolved oxygen during the summer and early fall controls profundal continental ostracode distribution in Crystal Lake (McHenry County), Illinois, favoring Cypria ophthalmica and Physocypria globula at water depths from 6 to 13 m. These species also thrived in the lake's profundal zone from 14,165 to 9600 calendar year before present (cal yr B.P.) during the late Bølling, Allerød, and Younger Dryas chronozones, and early Holocene. Characterized by sand, cemented tubules, large aquatic gastropod shells, and littoral ostracode valves, thin (1– 6 cm) tempestite deposits punctuate thicker deposits of organic gyttja from 16,080 to 11,900 cal yr B.P. The succeeding 2300 yr (11,900–9600 cal yr B.P.) lack tempestites, and reconstructed water depths were at their maximum. Deposition of marl under relatively well‐oxygenated conditions occurred during the remainder of the Holocene until the arrival of Europeans, when the lake returned to a pattern of seasonally low dissolved oxygen. Such conditions are also indicated in the lake sediment by the speciation of phosphorus, high concentrations of organic carbon, and abundant iron and manganese occluded to mineral grains. Initial low dissolved oxygen was probably caused by the delivery of dissolved P and Fe in shallow groundwater, the chemistry of which was influenced by Spodosol pedogenesis under a spruce forest. The triggering may have been regionally warm and wet conditions associated with retreat of the Lake Michigan lobe (south‐central Laurentide Ice Sheet).

Late Quaternary palaeohydrological changes in the large playa Lake Frome in central Australia, recorded from the Mg/Ca and Sr/Ca in ostracod valves and biotic remains

The last 42,000 years of hydrological history of Lake Frome, a large playa located in the arid part of northern South Australia, which is hypersaline and most often dry today, is reconstructed using a combination of ostracod assemblages, other microfossil remains, and the trace elemental composition of the selected halobiont ostracod species of Diacypris and Reticypris. The Mg/Ca and Sr/Ca of ostracod valves from 2 cores relate to significant hydrological changes that affected the lake over time. The reconstruction of the Sr/Ca of the lake’s waters, based on the Sr/Ca of ostracod shells, shows that when the lake fills the waters originate mostly from runoff, not from hypersaline waters located below the lake or the surrounding aquifers. The Last Glacial Maximum saw gypsum deflation from the lake. Prior to 25K yBP, Frome had a stable hydrological regime, permanent water and low salinities, with occasional freshwater conditions between 42 and 33K yBP. From 25 to 20.3K yBP, salinities fluctuated and ephemeral conditions operated. After that, until ∼14.8K yBP, a brine pool was located below the lake and was therefore under a different hydrological regime. Between 13 and 11.2K yBP, wet conditions occurred, but such conditions were not seen again during the Holocene.

6000 years of environmental changes recorded in Blue Lake, South Australia, based on ostracod ecology and valve chemistry

A 4 m long core taken from the freshwater Blue Lake crater near the township of Mount Gambier in southeastern South Australia provided a high-resolution palaeoclimatic record for the last six millennia. Accelerator Mass Spectrometry (AMS) radiocarbon ( 14C) dates were obtained from organic plant fibres and biogenic carbonates from the laminated sequence of the core and from a modern water sample. Large discrepancies between the radiocarbon ages determined from plant fibres and biogenic carbonates indicate the presence of a time-variable lacustrine reservoir, which is consistent with what is known of the lake's hydrology. Ostracod assemblages, associated with stable isotope (δ 13C, δ 18O) analyses and, in combination with Mg/Ca, Sr/Ca and Na/Ca analyses done on ostracod valves, infer salinity, temperature and water level changes in Blue Lake over the last 6 millenia. The influence of local aquifers through time has also been determined from the Na/Ca of ostracod valves. Approximately 900 year cycles are evident in the δ 13C record from 5.4 ka to 1.8 ka. The history of Blue Lake records an initial period of high hydrological variability around 6 ka, becoming increasingly deeper as groundwater flowed into the basin. By 4 ka, the lake had reached steady state with the lake level fluctuating by as much as 9 m, although significant geochemical variations represent temperature fluctuations until European settlement near the lake in 1839.

Multi-proxy evidence for early to mid-Holocene environmental and climatic changes in northeastern Poland

We investigated the sedimentary record of Lake Hancza (northeastern Poland) using a multi-proxy approach, focusing on early to mid-Holocene climatic and environmental changes. AMS 14C dating of terrestrial macrofossils and sedimentation rate estimates from occasional varve thickness measurements were used to establish a chronology. The onset of the Holocene at c. 11 600 cal. a BP is marked by the decline of Lateglacial shrub vegetation and a shift from clastic-detrital deposition to an autochthonous sedimentation dominated by biochemical calcite precipitation. Between 10 000 and 9000 cal. a BP, a further environmental and climatic improvement is indicated by the spread of deciduous forests, an increase in lake organic matter and a 1.7 parts per thousand rise in the oxygen isotope ratios of both endogenic calcite and ostracod valves. Rising delta 18O values were probably caused by a combination of hydrological and climatic factors. The persistence of relatively cold and dry climate conditions in northeastern Poland during the first one and a half millennia of the Holocene could be related to a regional eastern European atmospheric circulation pattern. Prevailing anticyclonic circulation linked to a high-pressure cell above the retreating Scandinavian Ice Sheet might have blocked the influence of warm and moist Westerlies and attenuated the early Holocene climatic amelioration in the Lake Hancza region until the final decay of the ice sheet.

Alkalinity control on the partition coefficients in lacustrine ostracodes from Australia

The partition coeffi cients (K D ) guiding the uptake of Mg and Sr in the low-Mg calcite valves of ostracodes in Australian saline lakes have been reviewed in line with new information on the bicarbonate content and the ! Mg/! Ca of the ambient waters. Strongly alkaline and very high ! Mg/! Ca (>>20) waters cause the partition coeffi cients of ostracodes to differ signifi cantly from those found in other lakes with lower ! Mg/! Ca values. Water salinity does not affect the uptake of Mg or Sr in ostracode valves, whereas the ! HCO 3 ‐ of the water strongly correlates to K D [Sr], with high ! HCO 3 ‐ resulting in a signifi

A review of the utility of Mg/Ca and Sr/Ca ratios of ostracode valves as a tool for paleoenvironmental reconstructions

微小甲殻類の一種である貝形虫は低マグネシウム方解石の殻を持っており，この殻の微量元素量および，その比は古環境指標として利用されている．一般に海棲の貝形虫の殻のMg/Ca比は，生息場の水温と強い相関があり，淡水棲の貝形虫の殻のMg/Ca比およびSr/Ca比は生息場のMg, Sr量を反映している．既存の研究を総括した結果，次のことが明らかになった．1）続成作用による殻のMg/Ca比およびSr/Ca比の変化を評価する方法は確立されていない．2）殻のMg/Ca比およびSr/Ca比の分配係数の範囲は分類群，生息場，系統によって異なる．3）これらの分配係数の範囲は種内の成長ステージによって異なる場合がある．4）海棲種のMg/Ca比の分配係数から見積もられる水温は約2～4℃の誤差を伴う．5）殻内の微小領域でのMgの分布の不均一性は殻全体のMg/Ca比に影響する．貝形虫殻のMg/Ca比，Sr/Ca比をより精密な環境指標として確立するためには，生息環境のMg/Ca比，Sr/Ca比，続成作用や試料の前処理の厳密な評価，およびより多くの分類群のデータが必要である．

Isotope measurements of single ostracod valves and gastropod shells for climate reconstruction: evaluation of within-sample variability and determination of optimum sample size

Sediment cores from Lakes Punta Laguna, Chichancanab, and Pet,n Itza on the Yucatan Peninsula were used to (1) investigate "within-horizon" stable isotope variability (delta O-18 and delta C-13) measured on multiple, single ostracod valves and gastropod shells, (2) determine the optimum number of individuals required to infer low-frequency climate changes, and (3) evaluate the potential for using intra-sample delta O-18 variability in ostracod and gastropod shells as a proxy measure for high-frequency climate variability. Calculated optimum sample numbers ("n") for delta O-18 and delta C-13 in the ostracod Cytheridella ilosvayi and the gastropod Pyrgophorus coronatus vary appreciably throughout the cores in all three lakes. Variability and optimum "n" values were, in most cases, larger for C. ilosvayi than for P. coronatus for delta O-18 measurements, whereas there was no significant difference for delta C-13 measurements. This finding may be explained by differences in the ecology and life history of the two taxa as well as contrasting modes of calcification. Individual delta O-18 measurements on C. ilosvayi in sediments from Lake Punta Laguna show that samples from core depths that have high mean delta O-18 values, indicative of low effective moisture, display lower variability, whereas samples with low mean delta O-18 values, reflecting times of higher effective moisture, display higher variability. Relatively dry periods were thus consistently dry, whereas relatively wet periods had both wet and dry years. This interpretation of data from the cores applies to two important periods of the late Holocene, the Maya Terminal Classic period and the Little Ice Age. delta O-18 variability during the ancient Maya Terminal Classic Period (ca. 910-990 AD) indicates not only the driest mean conditions in the last 3,000 years, but consistently dry climate. Variability of delta C-13 measurements in single stratigraphic layers displayed no relationship with climate conditions inferred from delta O-18 measurements.

Influence of anthropogenic pollution on ostracod assemblages of Amurskii Bay, Sea of Japan

The species composition and distribution of ostracods were investigated at two sites with different pollution levels on the eastern coast of Amurskii Bay within the limits of Vladivostok City. A total of 41 species were found. In all, thirty-eight species (28 of them alive) were found at the first site between Krasnyi and Groznyi capes. Another twenty-seven species were found at a depth of 1.5–3 m in the phytal zone with a diversity of microbiotopes. As the bottom became increasingly silty and the depth increased, the number of species decreased. A total of 25 species (only 15 of them alive) were found at the mouth of the Vtoraya Rechka River, which is heavily polluted by municipal and industrial sewage discharge. No valves of ostracods were found in surface sediments on silts at a depth down to 5.5m. At 4 m, only 2 species were found alive on stones overgrown with Saccharina japonica. At 500 m from a sewage discharge site, few ostracod valves were found that seemed to have died recently. At a distance of over 1 km, an ostracod assemblage typical of the silty substrates of Amurskii Bay was found below a 7 m depth (21 species, 12 of them alive).

Late Quaternary transition from lacustrine to a fluvio-lacustrine environment in the north-western Nam Co, Tibetan Plateau, China

Holocene sediments from a 9 m high outcrop at the Gangyasang Qu's entry into Lake Nam Co, on the south-eastern Tibetan Plateau, were studied in a multiproxy approach. The lowermost part of the section is of early Holocene age and consists of varved sediments deposited in a proglacial lake. Geomorphological investigations of fossil beach ridges and fossil cliff lines in the littoral zone of Nam Co indicate a late Pleistocene lake level high of 29 m above the present level. A warming is visible in ostracode-rich silts deposited after 9.6 ka BP. An ostracode-based transfer function estimates a lake level 28 m higher than today from this time until about 5.2 ka BP. A late Pleistocene lake level high was accompanied by an increase in lake size, causing Nam Co to overflow into the lower course of Gangyasang Qu, where a bay with slack-water conditions formed. Between 4.2 and 1.3 ka BP the lake level was approximately 10 m higher than today. The progradation of the Gangyasang Qu delta into Nam Co and the decreasing lake level shifted the study site into a fluvio-lacustrine environment. The sandy character and relatively low numbers of ostracode valves in the topmost sediments indicate increased fluvial activity, possibly due to a temporary increase in flood frequency. This facies lasted at least until 1.3 ka BP, when the lake level of Nam Co dropped again and the river incised more than 10 m into the underlying fluvio-lacustrine and lacustrine sediments.

Climate controlled ostracod preservation in Lake Ohrid (Albania, Macedonia)

Lake Ohrid (Albania, Macedonia) is probably the oldest existing lake in Europe and thus of great interest for palaeo-environmental and genetic studies. Within more than 220 endemic faunal species known to live in the lake, ostracods are outstanding due to the potentially very long fossil record. They can provide geochemical information over a period exceeding the entire Quaternary and direct evidence for radiation. Here we discuss the ostracod record and the calcite concentration along a 10-m-long sediment core taken at 100 m water depth in the south-eastern, Albanian part of Lake Ohrid. Well preserved ostracod valves representing in-situ fauna only occur from 0 cm to 105 cm and from 476 cm to 933 cm core depth, corresponding to the last interglacial and the Holocene, respectively. Those periods are also characterized by high concentrations of endogen calcite. There is no significant difference in the faunal composition of the two periods separated by several tens of thousand years and we found non-calcitic ostracod remains, especially the strongly chitinised mandibles, throughout the glacial period. Therefore, we conclude that ostracods were present in the lake throughout the entire period represented by the sediment core, but with poor or lacking preservation of their calcite valves during the glacial period. Calcite precipitation and ostracod preservation were most likely inhibited by the complete interruption of Calcium supply from karsts springs, which were inactive during the glacial periods due to permafrost in large parts of the calcareous mountains.

Distributions of ostracod (Crustacea) biofacies on the continental shelf off south-east Trinidad, western central Atlantic Ocean, suggest the location of an offshore river-induced front within the Orinoco Plume

Thirty two seafloor samples were taken over an area of 3600 km2 off south-east Trinidad (water depths 38–102 m), north-east of the Orinoco Delta. A total of 7097 ostracod carapaces and valves from 37 species were picked from them. Total recovery was co-dominated byBairdoppilata dorsoangulata andBradleya ex gr.dictyon, with lesserRadimella ovata, Loxoconcha sp. 2sensuBreman, 1982 andCosta variabilicostata recticostata. Cluster analysis revealed four biofacies approximately perpendicular to the bathymetric contours. This arrangement is contrary to the usual pattern, in which biofacies parallel contours, and is suggested to reflect the occurrence of a shelfal, river-induced front entrained within the hypopycnal Orinoco Plume. The front marks where outflow from the Essequibo and Orinoco rivers abuts against surface water rich in outflow from the Amazon. The biofacies immediately under the front yielded particularly high proportions ofB. ex. gr.dictyon andB. dorsoangulata, but relatively fewLoxoconcha sp. 2. Application of SHE Analysis for Biozone Identification (SHEBI) indicated that within the biofacies there occur abundance biozone boundaries at ∼55m, 75 m and 100 m that separate areas with differing population structures. SHE Analysis for Community Structure Investigation (SHESCI) indicated that, although the species proportions within most abundance biozones have a logarithmic series distribution, the rate of increase in species richness as specimens were accumulated was lowest in an abundance biozone below the front.

Developmental trajectories in geographically separated populations of non-marine ostracods: morphometric applications for palaeoecological studies

Developmental trajectories refer here to patterns of size and shape changes in ostracod valves during the sequence of post-embryonic growth stages. The information obtained from the study of such patterns has significant applications for evolutionary biology and/or (palaeo) ecology. Using geometric morphometrics methods, we describe the developmental trajectories of three ostracod species with valves retrieved from the sediments of lakes Mondsee (Austria), Hancza (Poland) and Iseo (Italy). Size and shape data result in distinct developmental trajectories. Ontogenetic changes agree with predictions of Prizbram’s and Brooks’ laws. Patterns of valve shape change provide information which might be of use to the taxonomic definition of evolutionary lineages and to the development of studies of allometry and heterochrony in non-marine ostracods.

A 1,100-year palaeoenvironmental record inferred from stable isotope and trace element compositions of ostracode and plant caryopses in sediments of Cattle Pond, Dongdao Island, South China Sea

Abundant ostracode valves (Cyprinotus cingalensis) and caryopses of Urochloa paspaloides, a terrestrial grass, were well preserved in the lacustrine sediments of the Cattle Pond on Dongdao Island, South China Sea. Oxygen and carbon isotopes, and elements (Ca, Mg) were analyzed on ostracode valves and plant caryopses in this study. The δ18Oostracode and δ13Costracode exhibited a positive and statistically significant correlation, and showed a decreasing trend toward the top of sediment sequence with some fluctuations, indicating a gradual increase in effective moisture. The Mg/Ca ratios in the ostracode shells, generally used as a proxy for salinity changes in lake water, showed a trend toward lower values in the upper samples, reflecting a gradual decrease of salinity in the lake. The δ13Corg values in the plant caryopses of the upper 14 cm of sediment have lower values than those in the bottom sediments, suggesting less water and salinity stress. These results indicate increasing effective moisture and rainfall intensity over the past ∼1,100 years on Dongdao Island.

Sex ratio of sub-fossil Ostracoda (Crustacea) from deep lake habitats in northern Poland

The present study examined sex ratios in five lacustrine profundal species displaying sexual dimorphism in the shape and/or size of the carapace valves: Limnocytherina sanctipatricii, Leucocythere mirabilis, Candona neglecta, Fabaeformiscandona levanderi and Fabaeformiscandona protzi. Data derived from a survey of sub-fossil ostracod valves preserved in short (up to 0.5 m) sediment cores collected from the sub-littoral and profundal zones of 12 mainly mesotrophic lakes in northern Poland. Mean sex ratios in all species were female-biased (ranging from 56% to 77% of females in F. protzi and L. mirabilis, respectively) and significantly different from a 1:1 ratio. The goodness of fit to a 2:1 (female:male) sex ratio model, commonly applicable in freshwater ostracods, was also tested and the model was rejected except for F. levanderi. In the remaining four species the differences were still statistically significant, although in F. protzi an excess of males caused the obtained deviation from 2:1. Subsequently, the homogeneity of sex ratios among all samples taken from different sediment layers, cores and lakes was analysed for the five species. For the four species collected in more than one lake (i.e. all but L. mirabilis) sex ratios were highly heterogeneous among all studied samples hence we cannot state that there is one specific sex ratio for each of these species. However, the sex ratios of each species among samples from a particular lake tested separately were homogeneous, i.e. did not show significant changes through time (core depth) in all cases except L. sanctipatricii in one lake. Homogeneity of sex ratios among all species from all samples within a given lake was also examined, whenever more than one species was collected. The sex ratios appeared heterogeneous among species only in two lakes. The test was repeated when the studied species were divided into two taxonomic groups (belonging either to Cytheroidea or to Cypridoidea superfamily) and the result was the same. The obtained results could suggest that the unbalanced sex ratio of some of the studied species could be caused at least partly by environmental conditions.

Eocene‐Oligocene paleoceanographic changes in the stratotype section, Massignano, Italy: Clues from rock magnetism and stable isotopes

We have conducted high‐resolution paleomagnetic and rock magnetic studies, in addition to stable isotope analyses of the Massignano sedimentary section, which is the Global Stratotype Section and Point (GSSP) for the Eocene‐Oligocene boundary. Our research builds upon the many past studies of the Massignano section in seeking to understand the timing and nature of the paleoenvironmental variations that occurred during the transition for the Earth's climate system from greenhouse to icehouse. The new paleomagnetic results provide a refined magnetostratigraphy of the section and new age for the Eocene‐Oligocene boundary at 33.7 Ma. Abrupt and large alternations in magnetic, concentration, composition, and grain sizes that occur in the high‐resolution rock magnetic record are interpreted to be the result of rapid bimodal shifts in deep‐sea circulation that affect sediment sources or transport. We speculate that currents flowing through the gateway between the Atlantic and Indo‐Pacific Oceans may have turned on and off as the gateway was progressively closing, resulting in the deposition of two different assemblages of magnetic minerals at Massignano. Finally, stable isotope (δ18O and δ13C) data collected on ostracod valves also suggest significant changes in sea bottom circulation in the Neo‐Tethys Ocean at and about 2 m.y. before the Eocene‐Oligocene boundary.

Species, valve size, and pretreatment effects on δ18O and δ13C values of ostracod valves from Lake Qinghai, Qinghai–Tibet Plateau

Stable carbon and oxygen isotope ratios of lacustrine ostracod valves are widely used to deduce paleoenvironmental information. We tested the effects of sample pretreatment methods (methanol, deionised water and methanol, and hydrogen peroxide) on ostracod shell oxygen and carbon isotope compositions. In addition, we examined the stable isotope differences among ostracod valves of different sizes and between the two species ( Eucypris inflata and Limnocythere inopinata) present in Lake Qinghai. We found that different treatment methods have no significant effect on the measured C or O stable isotope ratios of ostracod valves, and therefore propose that pretreatment is not needed before isotope analysis. Oxygen isotope ratios of E. inflata were positively correlated with L. inopinata. This correlation suggests that the δ 18O values of the two species respond similarly to environmental change. The δ 13C values of E. inflata showed no correlation with L. inopinata, and E. inflata had consistently higher δ 13C values than L. inopinata. Valve length had no effect on measured δ 18O values for E. inflata, but δ 13C values increased with the decreasing shell length (for length classes: < 0.8 mm, 0.8–0.65 mm, 0.65–0.5 mm and < 0.5 mm). Our results suggest that species and size have very little effect on the oxygen isotope composition of ostracod valves, making it easy to work with ostracod samples when studying lake δ 18O history. In contrast, much more attention must be paid to the selection of species and size when using the carbon isotope composition of ostracods for palaeoenvironmental reconstruction.

A multi-proxy geochemical investigation of the early Paleocene (Danian) continental palaeoclimate at the Fontllonga-3 site (South Central Pyrenees, Spain)

Chronologically well constrained non-marine deposits across the Cretaceous–Tertiary boundary (KTb) are exceptionally rare. The Fontllonga section (Tremp Formation, South Central Pyrenees, Lleida, Spain) constitutes one of these rare global records. Stable isotope (δ 18O CO3 and δ 13C) analyses have been performed on the carbonate fraction of 29 samples from diverse skeletal micro-remains (charophyte gyrogonites, gastropod shells, ostracod valves and isolated skeletal remains of lepisosteids and pycnodonts) from the earliest Danian site, Fontllonga-3. A mean Ba/Ca water palaeotemperature of 28.0 ± 6.7 °C has been obtained from the ganoine of 25 lepisosteid scales. This mean palaeotemperature is comparable with the temperature tolerance range for extant relatives of fossil osteoglossiform fish found at Fontllonga-3, which require a temperature range of 24°–35 °C (mean annual temperature 27–30 °C) to survive. Using the temperature range provided by the Ba/Ca palaeothermometer (21.3–34.7 °C), it is possible to determine δ 18O water values from the isotopic content of charophyte gyrogonites, gastropod shells, ostracod valves and fish remains (mean δ 18O CO3 = − 5.00 ‰, σ = 0.21). δ 18O water values of between − 4.01 and − 0.95‰ (VSMOW) are calculated, which, when combined with (La/Yb) N versus (La/Sm) N plots, are in good agreement with the sedimentary interpretation of the site as an estuarine environment. Based upon a comparison with modern day meteorical conditions these isotopic values are relatively high for subtropical freshwaters, suggesting a low precipitation rate (amount effect, high evaporation rate and/or long residence time). When combined with other isotopic, palaeobotanical and mineralogical studies carried out in the Pyrenean and Tethys realms this first geochemical study of an early Paleocene site is consistent with there being a subtropical seasonally dry climate in the South Central Pyrenees at this time.