Tests Of Planktonic Foraminifera Research Articles

Calcium Carbonate Dissolution Triggered by High Productivity During the Last Glacial–Interglacial Interval in the Deep Western South Atlantic

Studies reconstructing surface paleoproductivity and benthic environmental conditions allow us to measure the effectiveness of the biological pump, an important mechanism in the global climate system. In order to assess surface productivity changes and their effect on the seafloor, we studied the sediment core SAT-048A, spanning 43–5 ka, recovered from the continental slope (1,542 m water depth) of the southernmost Brazilian continental margin, deep western South Atlantic. We assessed the sea surface productivity, the organic matter flux to the seafloor, and calcite dissolution effects, based on micropaleontological (benthic and planktonic foraminifers, ostracods), geochemical (benthic δ13C isotopes), and sedimentological data (carbonate and bulk sand content). Superimposed on the induced changes related to the last glacial–interglacial transition, the reconstruction indicates a significant and positive correlation between the paleoproductivity proxies and the summer insolation. From the reconstructed data, it was possible to identify high (low) surface productivity, high (low) organic matter flux to the seafloor, and high (low) dissolution rates of planktonic Foraminifera tests during the glacial (postglacial). Furthermore, within the glacial, enhanced productivity was associated with higher insolation values, explained by increased northeasterly summer winds that promoted meandering and upwelling of the nutrient-rich South Atlantic Central Water. Statistical analyses support the idea that productivity is the main cause for seafloor calcium carbonate dissolution, as opposed to changes in the Atlantic Meridional Overturning Circulation (at least for the 25–4 ka period). Further efforts must be invested in the comprehension and quantification of the total organic matter and biogenic carbonate burial during time intervals with an enhanced biological pump, aiming to better understand their individual roles.

FRAGMENT OR BROKEN? IMPROVING THE PLANKTONIC FORAMINIFERA FRAGMENTATION ASSESSMENT

ABSTRACT Planktonic foraminifera tests can suffer dissolution, which usually involves partial damage, weight loss, and fragmentation. Since planktonic foraminifera assemblages, consisting of different resistant/susceptible species, can be strongly modified by dissolution, it is imperative to quantify its effect. The fragmentation index proposed 50 years ago has been used widely to measure preservation of planktonic foraminifera tests, but calibrations to this method are necessary. Some revisions are based on assumptions, like a certain number of fragments produced by a unique test, which is then used to compare whole tests with the dissolution remains. Likewise, researchers do not agree on what they count and how they identify what they count. Here we present a standardized and less subjective method, called fragmentation intensity (FI), to better assess the fragmentation of planktonic foraminifera through image software analysis, which includes both fragmentation remains (fragments and broken tests) and their measured area and perimeter. When compared to calcium carbonate content, grain sand content, and planktonic foraminifera tests per gram of dry sediment, the FI method derived better correlation values than the broken and fragments indexes. Future studies, in varying oceanographic contexts, can test this method to improve confidence, and eventually possibly adapt the index into a proxy for calcium carbonate undersaturation.

Crucial Boundaries in the Development of the Late Cretaceous Biota of Planktonic Foraminifera in the Southern Indian Ocean

The article analyzes the author’s own data on the species composition of planktonic foraminifera tests from Upper Cretaceous sediments in the Indian Ocean, as well as from sections of the marginal shelf seas of Australia. Planktonic foraminifera species are grouped and arranged into a climate series. Analysis of the change in the systematic composition of foraminifera made it possible to identify periods of extreme and intermediate climatic conditions in the Late Cretaceous.

The Late Cretaceous Fucoid Marl of the Ropianka Formation in the Kąkolówka Structure (Skole Nappe, Outer Carpathians, Poland) – lithology and foraminiferal biostratigraphy

A 250-m-thick sedimentary succession dominated by siliciclastic deposits occurs in the Kąkolowka Structure of the Skole Nappe. The succession was deposited in the inner part of the Skole Basin during the Late Cretaceous. In position and age, it corresponds to the Kropivnik Fucoid Marl of the Wiar Member (a subdivision of the Ropianka Formation), which was comprehensively described in the external part of the Skole Nappe. In this study, the authors provide the first complete data set on the lithological development and biostratigraphy of the Kropivnik Fucoid Marl from the inner part of the Skole Nappe. The results are compared to previous data from the outer part of the Skole Nappe. In the Kropivnik Fucoid Marl of the Kąkolowka Structure, three main heterolithic facies associations are distinguished: shale-sandstone, marl-sandstone and sandstone-shale. The occurrences of hard, platy and soft marls within siliciclastic rocks are typical of the sections studied. The features observed indicate a turbiditic origin of the deposits studied, including the hard, platy marls. The allogenic material of the strata described includes the small, fragile tests of planktonic foraminifera, which were redeposited from the outer parts of the Skole Basin. Particularly large concentrations of planktonic foraminifera were observed in the hard, platy marls. They are less common in the soft marls and shales. In the Zimny Dzial section, a diverse assemblage of benthic and planktonic foraminifera was found. The Kropivnik Fucoid Marl was dated as uppermost Campanian to lowermost Maastrichtian on the basis of planktonic foraminifera, which represent the Gansserina gansseri Zone. The agglutinated foraminiferal assemblages are representative for the lower part of the Rzehakina inclusa Zone and the co-occurrence of the Caudammina gigantean (Geroch) acme with Rzehakina inclusa (Grzybowski) was observed.

Temperature-dependent ocean–atmosphere equilibration of carbon isotopes in surface and intermediate waters over the deglaciation

High resolution and precise records of the carbon isotopic composition of atmospheric carbon dioxide from ice cores are now providing a valuable constraint on the origin of the rise in CO2 over the last deglaciation. Previous studies using isotopic measurements on the tests of planktonic foraminifera from marine sediments had shown a deglacial minimum in the carbon isotopic composition of seawater carbon in the surface and thermocline waters of the Southern Hemisphere and the tropics, a feature also seen in the atmospheric record. Here we show that this deglacial minimum also can be clearly seen in carbon isotope measurements on planktonic foraminifera from sediment cores in the North Atlantic. There are also new high-resolution and precise records of the carbon isotopic composition of intermediate and mode waters generated using benthic foraminifera which show a history of change that is strikingly similar to that in the atmosphere. Similarities in these new highly-resolved deglacial records suggest that air–sea exchange can effectively globalize the isotopic signature of a release of carbon from the deep ocean. This limits the utility of carbon isotopes for constraining the precise location and mechanism of carbon release, indicating that more direct nutrient tracers are needed. However, given that the equilibrium partitioning of carbon isotopes between the atmosphere and ocean is temperature dependent, the difference between the atmosphere and upper ocean values could potentially provide an independent estimate of sea surface temperature change.

Berriasian planktonic foraminifera and calcareous nannofossils from Crimea Mountains, with reference to microfossil evolution

A Berriasian age planktonic foraminifera assemblage from a section near the village of Krasnoselivka in the Tonas River Basin, Crimea contains Favusella hoterivica (Subbotina), ?Favusella sp., Conoglobigerina gulekhensis (Gorbachik and Poroshina), Lilliputinella eocretacea (Neagu), Lilliputinella aff. similis (Longoria), Hedbergella aff. handousi Salaj and ? Globuligerina sp. Specimens are poorly preserved, but test morphology, aperture and key wall texture features are recognizable. Age assignment is based on a diverse and well-preserved calcareous nannofossils assemblage of upper zone CC2 and ammonites (Jacobi Zone). The nannfossils indicate an open marine environment of Tethyan affinity. Several of the planktonic foraminiferal taxa were not previously described from pre-Valanginian or Hauterivian strata. XRD analysis of the tests of benthic and planktonic foraminifera and micro-gastropods shows these to be calcitic in composition, also of those benthic and planktonic foraminifera that were deemed to be originally aragonitic in composition, indicating diagenetic changes in carbonate fractions. From detailed comparison to Jurassic planktonic foraminifera, two lineages are proposed from Late Jurassic into earliest Cretaceous: Globuligerina oxfordiana (Grigelis) to Favusella hoterivica (Subbotina) and Globuligerina balakhmatovae (Morozova) to Lilliputinella eocretacea (Neagu). The first lineage is a gradual change in wall sculpture from rugulose to reticulate and the second one a change to a lower spire, more ovate chambers of which the last one may be much larger in size and shift of the aperture out of the umbilicus. The meagre evolution of early planktonic foraminifera from Toarcian through Tithonian with only two genera and fewer than ten species does not follow the evolutionary diversity pattern of nannofossils and dinoflagellates. Tithonian appears to be a bottleneck for planktonic foraminifera, with a sparse record and virtual extinction. The so-called Globigerina oxfordiana from the Tithonian in ODP Site 901 on Galicia Bank (Collins et al. Proceeding Deep Sea Drilling Project Science Research 149:193–201, 1996) is of Miocene age.

Foraminiferal Solubility Rankings: a Contribution To the Search For Consensus

Abstract Factors intrinsic to foraminiferal tests result in different degrees of susceptibility to dissolution. Differential resistance to dissolution among planktonic foraminifera has been studied in several regions, but no previous work has been reported for the western South Atlantic. The goal of this research was to develop a dissolution susceptibility ranking for planktonic foraminifera from the western South Atlantic Ocean, to compare the solubility between benthic and planktonic foraminifera, and to evaluate changes in the oxygen isotopic signal (δ18O) associated with dissolution. Two experiments were carried out by immersing tests in acetic acid or distilled water for 200 days. Our comparison revealed that tests of planktonic foraminifera were more resistant to dissolution than the benthic species tested, which has implications for use of the planktonic/benthic ratio (P/B) as a preservation proxy. Solubility of tests is directly proportional to the Mg content in the calcite structure, which varies widely across benthic taxa but is consistently low in planktonics. The δ18O increased during dissolution, probably due to the preferential dissolution of the internal chambers, making the remaining calcite progressively reflect the composition of the last chambers. Our solubility ranking for planktonic foraminifera agreed in part with rankings for foraminifera from other regions. Neogloboquadrina pachyderma and Globorotalia inflata proved to be resistant species; Globigerinella calida, Globigerinella siphonifera, Globorotalia hirsuta, Candeina nitida, and Trilobatus trilobus were moderately resistant; and Globorotalia fimbriata and Orbulina universa were most susceptible to dissolution. Presence and condition of benthic genera such as Quinqueloculina, Hoeglundina and Bulimina, and the planktonic species O. universa can be used to indicate limited dissolution in paleoceanographic studies. Small variations in the dissolution index may also reflect oceanographic changes that influence the composition of the tests, indicating the importance of regional solubility rankings, such as this study for the western South Atlantic.

High-resolution radiocarbon records trace episodes of Zoophycos burrowing

The unknown Zoophycos-producing organism potentially reworks marine sediments far deeper (up to >1 m) than common bioturbational mixing of the topmost sediment layer. The Zoophycos producer displaces planktonic foraminifera tests down to deep sediment levels hence, may seriously bias the faunal and isotopic composition and in particular, the radiocarbon age (14C) of the ambient host sediment. Here we present three high-resolution 14C records from sediment cores retrieved at intermediate water depths off Brazil, New Zealand, and Norway where Zoophycos burrows have biased 14C ages by 1000–5000 years. Based on closely spaced 14C ages and 14C plateau tuning we distinguish particular episodes with enhanced Zoophycos production that coincide with time spans of decreased sedimentation and thus, with probably reduced benthic nutrition. Off New Zealand, the actual burrowing activity was dated close to the Bølling/Allerød–Younger Dryas boundary. Displaced grains are traced from the ancient sediment surface of the Younger Dryas as far back as to the onset of Heinrich Stadial 1 and the terminal Last Glacial Maximum. These displaced 14C ages and similar 14C records obtained from two other sediment cores suggest that Zoophycos producers generally mount a burrowing campaign during times when environmental conditions are switching toward a starved sedimentation regime. During these tipping points Zoophycos spreiten probably serve as storage space for nutrient-rich particles foraged at the sediment surface and bioturbational mixed layer.

Effect of preservation state of planktonic foraminifera tests on the decrease in Mg/Ca due to reductive cleaning and on sample loss during cleaning

Four species of planktic foraminifera from core-tops spanning a depth transect on the Ontong Java Plateau were prepared for Mg/Ca analysis both with (Cd-cleaning) and without (Mg-cleaning) a reductive cleaning step. Reductive cleaning caused etching of foraminiferal calcite, focused on Mg-rich inner calcite, even on tests which had already been partially dissolved at the seafloor. Despite corrosion, there was no difference in Mg/Ca of Pulleniatina obliquiloculata between cleaning methods. Reductive cleaning decreased Mg/Ca by an average (all depths) of ~4% for Globigerinoides ruber white and ~10% for Neogloboquadrina dutertrei. Mg/Ca of Globigerinoides sacculifer (above the calcite saturation horizon only) was 5% lower after reductive cleaning. The decrease in Mg/Ca due to reductive cleaning appeared insensitive to preservation state for G. ruber, N. dutertrei and P. obliquiloculata. Mg/Ca of Cd-cleaned G. sacculifer appeared less sensitive to dissolution than that of Mg-cleaned. Mg-cleaning is adequate, but SEM and contaminants (Al/Ca, Fe/Ca and Mn/Ca) show that Cd-cleaning is more effective for porous species. A second aspect of the study addressed sample loss during cleaning. Lower yield after Cd-cleaning for G. ruber, G. sacculifer and N. dutertrei confirmed this to be the more aggressive method. Strongest correlations between yield and Δ[CO32−] in core-top samples were for Cd-cleaned G. ruber (r=0.88, p=0.020) and Cd-cleaned P. obliquiloculata (r=0.68, p=0.030). In a down-core record (WIND28K) correlation, r, between yield values >30% and dissolution index, XDX, was −0.61 (p=0.002). Where cleaning yield <30% most Mg-cleaned Mg/Ca values were biased by dissolution.

Antarctic ice growth before and after the Eocene‐Oligocene transition: New estimates from clumped isotope paleothermometry

AbstractAcross the Eocene‐Oligocene transition, the oxygen isotopic composition (δ18O) of benthic and planktonic foraminifera increased by over 1‰. This shift is thought to represent a combination of global cooling and the growth of a large ice sheet on the Antarctic continent. To determine the contribution of each of these factors to the total change in δ18O, we measured the clumped isotopic composition of planktonic foraminifera tests from Ocean Drilling Program Site 689 in the Southern Ocean. Near‐surface temperatures were ~12°C in the intervals 0–1.5 Myr before and 1–2 Myr after the major (Oi‐1) transition, in agreement with estimates made using other proxies at nearby sites. Temperatures cooled by 0.4 ± 1.1°C between these intervals, indicating that the long‐term change in δ18O seen in planktonic foraminifera at this site is predominantly due to changes in ice volume. A larger instantaneous cooling may have occurred during Oi‐1 but is not captured in this study due to sampling resolution. The corresponding change in the isotopic composition of seawater (δ18Osw) is 0.75 ± 0.23‰, which is within the range of previous estimates, and represents global ice growth equivalent to roughly ~110–120% of the volume of the modern Antarctic ice sheet or ~80–90 m of eustatic sea level change.

Evolutionary ecology of Early Paleocene planktonic foraminifera: size, depth habitat and symbiosis

The carbon stable isotope (δ13C) composition of the calcitic tests of planktonic foraminifera has an important role as a geochemical tracer of ocean carbon system changes associated with the Cretaceous/Paleogene (K/Pg) mass extinction event and its aftermath. Questions remain, however, about the extent of δ13C isotopic disequilibrium effects and the impact of depth habitat evolution on test calcite δ13C among rapidly evolving Paleocene species, and the influence this has on reconstructed surface-to-deep ocean dissolved inorganic carbon (DIC) gradients. A synthesis of new and existing multispecies data, on the relationship between δ13C and δ18O and test size, sheds light on these issues. Results suggest that early Paleocene species quickly radiated into a range of depths habitats in a thermally stratified water column. Negative δ18O gradients with increasing test size in some species ofPraemuricasuggest either ontogenetic or ecotypic dependence on calcification temperature that may reflect depth/light controlled variability in symbiont photosynthetic activity. The pattern of positive δ13C test-size correlations allows us to (1) identify metabolic disequilibrium δ13C effects in small foraminifera tests, as occur in the immediate aftermath of the K/Pg event, (2) constrain the timing of evolution of foraminiferal photosymbiosis to 63.5 Ma, ∼0.9 Myr earlier than previously suggested, and (3) identify the apparent loss of symbiosis in a late-ranging morphotype ofPraemurica. These findings have implications for interpreting δ13C DIC gradients at a resolution appropriate for incoming highly resolved K/Pg core records.

Quantifying sea surface temperature ranges of the Arabian Sea for the past 20 000 years

Abstract. The oxygen isotopic composition of planktonic foraminifera tests is one of the widest used geochemical tools to reconstruct past changes of physical parameters of the upper ocean. It is common practice to analyze multiple individuals from a mono-specific population and assume that the outcome reflects a mean value of the environmental conditions during calcification of the analyzed individuals. Here we present the oxygen isotope composition of individual specimens of the surface-dwelling species Globigerinoides ruber and Globigerina bulloides from sediment cores in the Western Arabian Sea off Somalia, inferred as indicators of past seasonal ranges in temperature. Combining the δ18O measurements of individual specimens to obtain temperature ranges with Mg/Ca based mean calcification temperatures allows us to reconstruct temperature extrema. Our results indicate that over the past 20 kyr the seasonal temperature range has fluctuated from its present value of 16 °C to mean values of 13 °C and 11 °C for the Holocene and LGM, respectively. The data for the LGM suggest that the maximum temperature was lower, whilst minimum temperature remained approximately constant. The rather minor variability in lowest summer temperatures during the LGM suggests roughly constant summer monsoon intensity, while upwelling-induced productivity was lowered.

Relationship between isotopic composition (Δ18O and Δ13C) and plaktonic foraminifera test size in core tops from the Brazilian Continental Margin

Stable oxygen (δ18O) and carbon (δ13C) isotopic signature registered in fossil planktonic foraminifera tests are widely used to reconstruct ancient oceanographic conditions. Test size is a major source of stable isotope variability in planktonic foraminifera found in sediment samples and thus can compromise paleoceanographic interpretations. Test size/stable isotope (δ18O and δ13C) relationships were evaluated in two planktonic foraminifer species (Globigerinoides ruber (white) and Globorotalia truncatulinoides (right)) in two core tops from the Brazilian Continental Margin. δ18 Omeasurements were used to predict the depth of calcification of each test size fraction. δ13C offsets for each test size fraction were then estimated. No systematic δ18O changes with size were observed in G. ruber (white) suggesting a similar calcification depth range (c.a. 100 m) during ontogeny. For G. truncatulinoides (right) δ18O values increased with size indicating ontogenetic migration along thermocline waters (250-400 m). δ13C measurements and δ13C offsets increased with size for both species reflecting well known physiological induced ontogenetic-related variability. In G. ruber (white) the largest test size fractions (300µm and &gt;355µm) more closely reflect δ13C DIC indicating they are best suited for paleoceanographic studies.

Improving temperature estimates derived from Mg/Ca of planktonic foraminifera using X‐ray computed tomography–based dissolution index, XDX

Temperatures derived from Mg/Ca ratios of the calcite tests of planktonic foraminifera are distorted when samples are partially dissolved, and methods are required to quantify this source of inaccuracy. Here we compare a dissolution index (XDX), based on X‐ray computed tomography scans, to Mg/Ca of four species of foraminifera (G. ruber (white), G. sacculifer (without sac), N. dutertrei, and P. obliquiloculata) from core top sediments from the tropical Pacific, Atlantic, and western Indian Ocean. Deepwater calcite saturation values (Δ[CO32−]) at the sites ranged from 55 to −23 μmol/kg. An estimate of ΔMg/Ca (reduction in Mg/Ca due to dissolution) was made for each sample. ΔMg/Ca decreased linearly from deepwater Δ[CO32−] values of between 10 (±4) and 15 (±5) μmol/kg. These values are minimum estimates of the threshold below which Mg/Ca is affected by dissolution, as they are limited by assumptions made in calculating ΔMg/Ca. Sensitivity of Mg/Ca to Δ[CO32−] was greatest for G. ruber, where Mg/Ca decreased by 0.102 (±0.036) mmol/mol per μmol/kg. Sensitivity was similar for G. sacculifer (0.047 ± 0.015 mmol/mol per μmol/kg), N. dutertrei (0.037 ± 0.010 mmol/mol per μmol/kg), and P. obliquiloculata (0.040 ± 0.008 mmol/mol per μmol/kg). Sensitivity was similar at all sites for each species, excepting an apparently greater response for N. dutertrei from the Caribbean compared to other sites. Calibrations between XDX and ΔMg/Ca provide a means to estimate dissolution bias on Mg/Ca. Poor correlation between XDX and δ18O suggests that, for the small sample size typical for analysis, variability in initial δ18O overwhelms dissolution effects.

Stable isotopes in modern planktonic foraminifera: Sediment trap and plankton tow results from the South China Sea

Abstract This study reports on the stable isotopic composition of modern planktonic foraminifera tests collected from plankton tows and sediment trap moorings in the northern South China Sea. Plankton tow samplings were conducted at water depths of 50, 100, and 200 m during various seasons between December 2002 and August 2008. The sediment trap array was deployed between August 2004 and February 2005 at water depths of 121 m, 619 m, and 3451 m. Four common and widely distributed tropical/subtropical planktonic foraminifer species, including Globigerinoides ruber (white variety), Globigerinoides sacculifer (without sac), Neogloboquadrina dutertrei and Pulleniatina obliquiloculata, were analyzed for δ18O and δ13C within narrow shell size ranges and compared with the concurrent sea surface temperature (SST) and wind stress. Our results show that foraminiferal δ18O is primarily influenced by seawater temperature, while δ13C is affected by surface water nutrients, which in this region can be discerned from wind stress data. The δ18O and δ13C values from foraminifers recovered in plankton tows generally show greater variability and more depleted δ18O than values of tests collected by sediment traps. For foraminifer tests collected by sediment trap, the δ18O of the shallow dwelling G. ruber shows the largest amplitude variation (ca. 1.6‰) among the four species. In contrast, the δ13C of P. obliquiloculata shows relatively constant values throughout the study period. A pattern of enriched δ18O values, associated with marked δ13C depletion, is common to three species collected between late October and late December 2004. This distinct isotopic signal corresponds to a decrease in SST and increase in wind stress, indicating the onset of prevailing northeast winds during the winter season.

Inside story: An X-ray computed tomography method for assessing dissolution in the tests of planktonic foraminifera

X-ray computed tomography (CT) provides an insight into the progression of dissolution in the tests of planktonic foraminifera. Four species of foraminifera (G. ruber [white], G. sacculifer, N. dutertrei and P. obliquiloculata) from Pacific, Atlantic and Indian Ocean core-top samples were examined by CT and SEM. Inner chamber walls began to dissolve at Δ[CO32−] values of 12–14 μmol/kg. Close to the calcite saturation horizon, dissolution and precipitation of calcite may occur simultaneously. Inner calcite of G. sacculifer, N. dutertrei and P. obliquiloculata from such sites appeared altered or replaced, whereas outer crust calcite was dense with no pores. Unlike the other species, there was no distinction between inner and outer calcite in CT scans of G. ruber. Empty calcite crusts of N. dutertrei and P. obliquiloculata were most resistant to dissolution and were present in samples where Δ[CO32−] ≈ − 20 μmol/kg. Five stages of preservation were identified in CT scans, and an empirical dissolution index, XDX, was established. XDX appears to be insensitive to initial test mass. Mass loss in response to dissolution was similar between species and sites at ~ 0.4 μg/μmol/kg. We provide calibrations to estimate Δ[CO32−] and initial test mass from XDX.

Dissolution effects on the crystallography and Mg/Ca content of planktonic foraminifera Globorotalia tumida (Rotaliina) revealed by X‐ray diffractometry

Several authors suggested that the thinning, with increasing depth of deposition, of calcite X‐ray diffractometry (XRD) peaks obtained on planktonic foraminifera tests resulted from the preferential removal of their poorly crystallized parts as dissolution increases. By deconvolving XRD peak (104) from Globorotalia tumida (surface sediments, Sierra Leone Rise depth transect), we show that the full width at midheight does not depend only upon crystallinity, but reflects also the chemical and structural heterogeneity of foraminifera tests, which results in closely spaced, individual (104) diffraction peaks corresponding to phases with slightly different Mg contents. G. tumida contains two calcite phases: a well crystallized, Mg‐poor calcite and a poorly crystallized, Mg‐richer calcite. Increasing dissolution results in the preferential removal of the Mg‐richer calcite and the improvement of its crystallinity, whereas the Mg‐poor calcite does not seem to be affected by dissolution between 2750 m and 4950 m of water depth on Sierra Leone Rise.

In situ measurements of Li isotopes in foraminifera

In situ measurement of Li isotope ratios in foraminifera has been developed using a Cameca ims 1270 ion microprobe. In situ δ7Li analyses have been performed in biogenic calcite of planktonic foraminifera from various locations. Results show that for west Pacific mixed Globigerinoides and Globorotalia (22°S161°E), the isotopic variability between tests and within a single test, respectively, is not significantly greater than estimated analytical uncertainty (∼1.5‰). Mean δ7Li for several planktonic foraminifera tests corresponds to the seawater value, strongly suggesting negligible Li isotope fractionation relative to seawater, as previously inferred by Hall et al. (2005) using thermo‐ionization mass spectrometer and multicollector‐inductively coupled plasma‐mass spectrometry techniques. Combined with scanning electron microscopy and ion microprobe imaging, micron‐sized grains, enriched in lithium, silica and aluminum have been found in the foraminifera calcite matrix. A simple mixing model shows that 0.3–2 wt % of marine clays incorporated within the analyzed calcite would lower the foraminifera δ7Li value, by 3‰ to 10‰ relative to the isotopic composition of the pure calcite. By comparison, no such grains have been detected in corals. The presence of micron‐sized silicate grains embedded within the foraminifera calcite is consistent with the Erez (2003) biomineralization model, involving calcite precipitation from seawater vacuoles. By contrast, coral calcium carbonate is instead precipitated from ions, which have been pumped or diffused through several membranes, impermeable to micrometric grains. Ion microprobe in situ δ7Li measurements in biogenic calcite present new methods for investigating both biomineralization processes and the past record of the ocean composition by exploring geochemical variations at a scale that is smaller in space and in time.

Mg/Ca variation in planktonic foraminifera tests: implications for reconstructing palaeo-seawater temperature and habitat migration

The nature of compositional variability within the tiny calcitic shells (tests) that are precipitated by planktonic foraminifera has been investigated using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Systematic large and correlated variation of Mg/Ca, Mn/Ca, Ba/Ca and Zn/Ca but relatively uniform Sr/Ca are observed through the test walls of analysed species ( Globigerinoides sacculifer, Globigerinoides ruber, Neogloboquadrina pachyderma and Neogloboquadrina dutertrei). Distinct chamber and chamber-wall layer compositions can be resolved within individual tests, and Mg/Ca compositional differences observed in sequentially precipitated test components of the different species analysed are consistent with seawater temperature changes occurring with habitat migration during their adult life-cycle stages. Estimated test calcification temperatures are in keeping with available seawater temperature constraints, indicating the potential for accurate seawater temperature reconstruction using LA-ICP-MS. Mg-rich (<1–6 mol% Mg) surface veneers that are also enriched in Mn, Ba, and Zn have been found on all species and all fossil tests, as well as on live-sampled tests of G. ruber, with the latter suggesting a possible biogenic origin. These Mg-rich surfaces bias bulk test compositions toward higher Mg/Ca values by between 5 and 35%.

Late Pleistocene nutrients and sea surface productivity in the South China Sea: a record of teleconnections with Northern hemisphere events

Surface nutrient variations during the last 200,000 years have been reconstructed based on the Cd/Ca ratios in the tests of planktonic foraminifera ( Neogloboquadrina dutertrei) and the biogenic opal content from two gravity cores retrieved from the northern and northwestern parts of the South China Sea (SCS). The Cd/Ca ratios obtained from N. dutertrei in Recent sediments from fourteen box cores distributed over the SCS basin reflect present nutrient levels in the overlying surface waters. Areas with relatively high Cd/Ca values in the surface sediments are in the region of high chlorophyll-a in the northern South China Sea near the Dongsha Islands, consistent with previous observation. High values of Cd/Ca found in sediments east of Vietnam and east of the Nansha Islands may be a result of increased nutrients due to local upwelling induced by the summer monsoon. Core 17940-2, raised from the northern continental slope of the SCS, exhibits exceptional high sedimentation rates (25–85 cm/kyr) for the last 35 kyr. The Cd/Ca ratios in this core are relatively higher during the periods of late δ 18O stage 3 and the mid-Holocene. Core 17950-2, with an average sedimentation rate of 5 cm/kyr, offers a continuous long record back to δ 18O stage 7. In general, Cd/Ca values in the downcore records are higher during interglacials and progressively increase as the interglacials proceed. Values peak around the times of optimum climate conditions, and correlate with periods of increased opal wt%. Correlations in the interval of overlap between the two records support the interpretation that variations in Cd/Ca ratios in core 17940-2 are not an individual signal but can be replicated. Of special interest in this study is the tentative correlation between Cd/Ca peaks in the SCS record and the timing of iceberg discharge events (Heinrich events) in the North Atlantic. Mass accumulation rate (MAR) of opal data from core 17950-2 indicate high surface productivity during glacial intervals corresponding to oxygen isotope stages 6, 4, and 2. The increase in surface productivity during glacials from the northern SCS is contemporaneous with an intensified winter monsoon as recorded by the terrestrial loess deposits. The agreement with wind strength recorded in the Loess Plateau from mainland China supports the connection between the marine opal records of the SCS and paleomonsoon intensity, and thus a continental–marine link.