Core-top Samples Research Articles

The Zn abundance and isotopic composition of diatom frustules, a proxy for Zn availability in ocean surface seawater

We have developed cleaning methods for extracting diatomopal from bulk marine sediment samples, for measurement of both zinc (Zn) abundance and isotope composition. This cleaning technique was then applied to a set of Holocene core-top samples from the Southern Ocean. The measured δ66Zn (reported relative to the JMCLyon standard) and Zn/Si ratios from the Southern Ocean diatomopal samples range from 0.7 to 1.5‰, and from 14 to 0.9μmol/mol, respectively. The Zn abundance and isotope composition data show a clear correlation with opal burial rates and other oceanographic parameters. In common with previous work, we interpret the systematic changes in the Zn/Si ratio to be linked to the variability in the concentrations of bioavailable Zn in the ambient surface seawater where the diatom opal is formed. This variability is likely to be primarily controlled by the degree to which Zn is taken up into phytoplankton biomass. The observed systematic pattern in the δ66Zn compositions of the diatomopal core-top samples is, similarly, likely to reflect changes in the δ66Zn composition of the ambient Zn in the surface waters above the core-top sites, which is progressively driven towards isotopically heavier values by preferential incorporation of the lighter isotopes into phytoplankton organic material. Thus, the systematic relationship between Zn isotopes and abundance observed in the core-top diatomopal samples suggests a potential tool for investigating the biogeochemical cycling of Zn in the past surface ocean for down-core diatomopal material. In this respect, it may be possible to test hypotheses that attribute variations in atmospheric CO2 on glacial–interglacial timescales to the degree to which trace metals limited primary productivity in HNLC zones.

Paleoenvironmental changes in the northern South China Sea over the past 28,000 years: A study of TEX 86-derived sea surface temperatures and terrestrial biomarkers

We have generated a record of TEX 86 ( TEX 86 H )-derived sea surface temperatures (SSTs) over the last 28 ka for core MD97-2146 from the northern South China Sea (SCS). The TEX 86 H -derived temperature of a core-top sample corresponds to the SSTs in warmer seasons. The SST record shows a drop during the Oldest Dryas period, an abrupt rise at the onset of the Bølling–Allerød period, a plateau across the Younger Dryas period, and an abrupt rise at the beginning of the Holocene. The glacial–interglacial contrast in TEX 86 H -derived temperature is almost the same as that in foraminiferal Mg/Ca ratio-derived temperature, but it is larger than those in U 37 K ′ and transfer function-derived temperatures. Possible interpretations are: (1) the seasonal shift of glycerol dialkyl glycerol tetraether (GDGT) production, (2) the overestimation of temperature change by TEX 86 H , and (3) the underestimation of temperature change by U 37 K ′ and transfer function approaches. The similar variation in TEX 86 H -derived temperature at the study site and Chinese stalagmite δ 18O during the last deglaciation suggests that changes in TEX 86 H -derived temperature in this period reflected atmospheric and oceanic reorganization on a millennial timescale. The long-chain n-alkanes are mainly of higher plant origin before ∼14 ka and a mixture of higher plant and lithic origins after ∼14 ka; the abundance ratio of long-chain to short-chain n-fatty acids decreases at ∼15 ka, suggesting a drastic change in sediment sources at ∼14–15 ka. We attribute the higher content of fresh higher plant n-alkanes and long-chain n-fatty acids before ∼14–15 ka to enhanced aeolian transportation and/or arid environments. Increased precipitation likely due to intensified summer monsoon after ∼14–15 ka enhanced the erosion of sedimentary rocks and increased the contribution of lithic n-alkanes.

Progress toward a multi-basin calibration for quantifying deep sea calcite preservation in the tropical/subtropical world ocean

The development of a quantitative and multi-basin deep sea calcite preservation proxy is important for a better understanding of the marine carbonate system, both in modern sediments and in down core work. We present CaCO 3 preservation data from 89 core top samples in tropical and subtropical areas of the Pacific, Atlantic and Indian Oceans. We observe that the Globorotalia menardii Fragmentation Index (MFI) has potential to trace deep sea CaCO 3 dissolution quantitatively across the tropical/subtropical world ocean. This is evident in the robust relationship between MFI and bottom water calcite under-saturation in six depth transects with markedly different bottom water CO 3 2− saturation states. We also found a strong relationship between MFI and modeled estimates of percent CaCO 3 dissolved at our core top locations. Corroboration for MFI's dissolution trend in the subtropics comes from the concurrent drop in size-normalized shell weight of Globorotalia truncatulinoides specimens with increasing CO 3 2− under-saturation, and the dissolution driven drop in Mg/Ca measurements from tests of three foraminifer species: G. truncatulinoides, Globigerina conglobatus and Globorotalia hirsuta. MFI's core top calibration is further corroborated by the fragmentation trend of foraminifers that prefer higher latitudes like Globigerina bulloides, G. truncatulinoides and Neogloboquadrina pachyderma. Our observations suggest that MFI is coming closer to meeting the criteria for an ideal calcite dissolution proxy with world-wide applicability.

Controls on the age of vascular plant biomarkers in Black Sea sediments

Transfer of organic carbon (OC) from the terrestrial to the oceanic carbon pool is largely driven by riverine and aeolian transport. Before transport, however, terrigenous organic matter can be retained in intermediate terrestrial reservoirs such as soils. Using compound-specific radiocarbon analysis of terrigenous biomarkers their average terrestrial residence time can be evaluated. Here we show compound-specific radiocarbon ( 14C) ages of terrigenous biomarkers and bulk 14C ages accompanied by geochemical proxy data from core top samples collected along transects in front of several river mouths in the Black Sea. 14C ages of long chain n-alkanes, long chain n-fatty acids and total organic carbon (TOC) are highest in front of the river mouths, correlating well with BIT (branched and isoprenoid tetraether) indices, which indicates contribution of pre-aged, soil-derived terrigenous organic matter. The radiocarbon ages decrease further offshore towards locations where organic matter is dominated by marine production and aeolian input potentially contributes terrigenous organic matter. Average terrestrial residence times of vascular plant biomarkers deduced from n-C 29+31 alkanes and n-C 28+30 fatty acids ages from stations directly in front of the river mouths range from 900 ± 70 years to 4400 ± 170 years. These average residence times correlate with size and topography in climatically similar catchments, whereas the climatic regime appears to control continental carbon turnover times in morphologically similar drainage areas of the Black Sea catchment. Along-transect data imply petrogenic contribution of n-C 29+31 alkanes and input via different terrigenous biomarker transport modes, i.e., riverine and aeolian, resulting in aged biomarkers at offshore core locations. Because n-C 29+31 alkanes show contributions from petrogenic sources, n-C 28+30 fatty acids likely provide better estimates of average terrestrial residence times of vascular plant biomarkers. Moreover, sedimentary n-C 28 and n-C 30 fatty acids appear clearly much less influenced by autochthonous sources than n-C 24 and n-C 26 fatty acids as indicated by increasing radiocarbon ages with increasing chain-length and are, thus, more representative as vascular plant biomarkers.

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.

Effects of seafloor and laboratory dissolution on the Mg/Ca composition of Globigerinoides sacculifer and Orbulina universa tests — A laser ablation ICPMS microanalysis perspective

Partial or selective dissolution of planktonic foraminiferal tests on the seafloor has been shown to alter original test Mg/Ca compositions and thus may limit the accuracy of Mg/Ca-based thermometry for reconstructions of past sea surface temperatures. We have employed laser ablation ICPMS to determine the extent of dissolution-caused changes in Mg/Ca distribution across individual chamber walls of the planktonic foraminifera Globigerinoides sacculifer and Orbulina universa. G. sacculifer samples collected from a core-top depth transect in the NE Indian Ocean and laboratory dissolution experiments show little if any evidence of preferential removal of Mg-rich calcite layers by progressive dissolution of the tests. We attribute the absence of selective dissolution to the banded distribution of Mg across the chamber walls of these foraminiferal species and to the minimal presence of calcite crusts with relatively low-Mg composition on the outer surfaces of tests. Mg/Ca microanalyses of G. sacculifer from core-top samples further indicate that for samples collected above the calcite lysocline the effect of postdepositional dissolution on Mg/Ca sample mean values is minimal and within the uncertainty of Mg/Ca thermometry (i.e. ± 0.4 mmol/mol; ± 0.8 °C at ∼ 28 °C). Comparison with previously published results for G. sacculifer supports these observations. Simple modelling of G. sacculifer test dissolution indicates that selective removal of calcite with high-Mg/Ca values from within the final chamber of G. sacculifer test appears insufficient to cause the ∼ 10% decrease in Mg/Ca values observed above calcite lysocline. These changes in test composition might be related to development/removal as a function of Δ[CO 3 2−] of a thin diagenetic surface coating which has a relatively high-Mg/Ca composition (i.e. 20–25 mmol/mol).

Seasonal production patterns of planktonic foraminifera in the NE Atlantic Ocean: Implications for paleotemperature and hydrographic reconstructions

[1] Sediment trap samples from OMEX 2 (49°N, 13°W) provide a continuous record of the seasonal succession of planktonic foraminifera in the midlatitude North Atlantic and reveal a complex relationship between periods of production and specific hydrographic conditions. Neogloboquadrina pachyderma dextral coiling (d.), Globigerina bulloides, and Globorotalia inflata are found in great numbers during both the spring and summer seasons, whereas Globigerina quinqueloba, Globorotalia hirsuta, Globorotalia scitula, and Globigerinita glutinata are associated predominantly with the increase in productivity during the spring bloom. Globigerinella aequilateralis, Orbulina universa, and Globigerinoides sacculifer are restricted to late summer conditions following the establishment of a warm, well-stratified surface ocean. An annually integrated fauna from the sediment trap, comprising ∼13,000 individuals, is used to evaluate the accuracy of five faunal-based statistical methods of paleotemperature estimation. All of the temperature reconstruction techniques produce estimates of ∼16°C and ∼11°C for summer and winter surface temperature, respectively, which are in excellent agreement with regional hydrographic data and suggest that the sediment trap assemblage is well represented in the core top faunas. Analysis of the key species that dominate the OMEX 2 sediment trap fauna, G. bulloides, G. inflata, and N. pachyderma d., based on δ18O derived temperatures from North Atlantic core top samples, suggests that seasonal variations in planktonic foraminiferal production are nonuniform across the midlatitudes and that this is likely to complicate reconstructing past seasonal hydrographic dynamics using these taxa.

Early to mid-Pliocene environmentally constrained diatom assemblages from the AND-1B drillcore, McMurdo Sound, Antarctica

In extended lower to mid-Pliocene stratigraphic record of near-shore environmental variation in the southwestern Ross Sea, Antarctica is preserved in ANDRILL’s McMurdo Ice Shelf Project AND-1B drillcore.A94meter-thick sequence of three diatomite and diatomaceous mudstone intervals represents the highest latitude record (78° S) of persistent open-marine conditions on the Antarctic continental shelf, in response to a protracted period of Pliocene warmth. This core provides new chronostratigraphic control for Antarctic paleoclimatic events, and describes a sequence of distinct diatom assemblages comprised of both extant and extinct species that are unknown on the Antarctic shelf today. Common taxa include those with modern distributions close to the Polar Frontal Zone (e.g. Shionodiscus tetraoestrupii, Stellarima stellaris, Thalassionema nitzschioides), suggesting marine temperatures in the southwestern Ross Sea warmer than at present. Representatives of the modern sea-ice assemblage (e.g. Fragilariopsis curta, Stellarima microtrias) and cold open-marine species associated with the retreating sea-ice front (e.g. Eucampia antarctica var. recta) are also present, but in much lower abundance than in the Ross Sea today as suggested by core top and surface sample data. The co-occurrence of species from these three distinct modern environments in Pliocene sediments, in close proximity to the continent, suggests that modern ecological zonal concepts cannot be applied directly in Pliocene paleoenvironmental analyses. Species present in the 4 m-thick lowest diatomite unit (~4.5 Ma) indicate a cold openmarine environment,with the period of deposition likely comprising the length of one interglacial period. The middle diatomite unit (~4.1-4.3 Ma; 14m) records warmer and perhaps seasonally mixed conditions with often abundant Thalassionema nitzschioides and Shionodiscus tetraoestrupii and little evidence of sea-ice associated species. Documentation of changes in diatom assemblages through the upper 75m-thick diatomite interval (~3.3-3.6Ma) offers the opportunity to resolve environmental limits of Pliocene diatom species thatwill serve as a guide to diatom-based paleoenvironmental interpretations for other Pliocene deposits. This upper unit records deposition during several glacial/interglacial cycles indicating an extended interval with open marine productivity during the mid-Pliocene. In addition, environmental associations for extinct species are inferred by their co-occurrence with extant taxa. Persistent intervals of pure diatomite suggest high productivity in an open-marine setting and minimal to no influence of sea ice. Commonly occurring species can be placed in five ecological cateories; Neritic/Stratified, Warm, Cold/Sea Ice, Heavy/Wind Mixed and Mixed. Comparison of this long diatom-bearing interval with coeval Antarctic deposits presents a regional and continental picture of Pliocene warmth, suggesting East Antarctic Ice Sheet was smaller with sub-polar marginal conditions.

Diatom (< 63 µm) distribution offshore of eastern New Zealand: Surface sediment record and temperature transfer function

Diatom microfossils preserved in 38 core top samples from offshore of eastern New Zealand are examined to develop the first offshore modern analogue dataset for diatoms in New Zealand waters. The sample set consists of seven, approximately E–W trending depth transects with samples from the continental shelf, slope and abyssal plain between latitudes 33° and 54°S. The identification of terrestrial and coastal species in open marine core top samples indicates that offshore transport of diatom microfossils occurs to an extent that could be useful for sediment provenance studies or at least to enable improved identification of transported versus in-situ deposition on the continental shelf. Cluster analysis and ordination indicate that the Chatham Rise and/or the overlying Subtropical Front are the most significant bathymetric/oceanographic features in the dataset as this location shows up as a major barrier or transition zone in terms of diatom distribution. The strongest environmental control on diatom distribution is the north–south sea surface temperature gradient. A statistically strong relationship ( r 2 = 0.81; RMSEP = 1.57) exists between diatom abundance and sea surface temperature making diatoms ideal for quantitative reconstruction of past sea surface temperatures in future paleoclimate studies.

Rapid climatic variability in the west Mediterranean during the last 25 000 years from high resolution pollen data

Abstract. High-temporal resolution pollen record from the Alboran Sea ODP Site 976, pollen-based quantitative climate reconstruction and biomisation show that changes of Mediterranean vegetation have been clearly modulated by short and long term variability during the last 25 000 years. The reliability of the quantitative climate reconstruction from marine pollen spectra has been tested using 22 marine core-top samples from the Mediterranean. The ODP Site 976 pollen record and climatic reconstruction confirm that Mediterranean environments have a rapid response to the climatic fluctuations during the last Termination. The western Mediterranean vegetation response appears nearly synchronous with North Atlantic variability during the last deglaciation as well as during the Holocene. High-resolution analyses of the ODP Site 976 pollen record show a cooling trend during the Bölling/Allerød period. In addition, this period is marked by two warm episodes bracketing a cooling event that represent the Bölling-Older Dryas-Allerød succession. During the Holocene, recurrent declines of the forest cover over the Alboran Sea borderlands indicate climate events that correlate well with several events of increased Mediterranean dryness observed on the continent and with Mediterranean Sea cooling episodes detected by alkenone-based sea surface temperature reconstructions. These events clearly reflect the response of the Mediterranean vegetation to the North Atlantic Holocene cold events.

A critical evaluation of calcium isotope ratios in tests of planktonic foraminifers

We critically evaluate the applicability of Ca-isotope ratios in planktonic foraminifers as proxy for past sea surface temperatures (SST) and isotope composition of paleo-seawater (δ 44Ca sw) reconstructions. Previous studies have shown discrepancies regarding the temperature sensitivity of Ca isotope fractionation in foraminifers of more than one order of magnitude. We present new data from the planktonic foraminifer species Orbulina universa, Globigerinoides sacculifer and Neogloboquadrina pachyderma (sinistral) from culture experiments, multinet deployments and coretop samples. Specimens of G. sacculifer cultured at low salinities (33–34.5) show predominantly no major temperature dependent Ca isotope fractionation, in contrast to previous individuals cultured at higher salinities of 34.5–36. The new data of O. universa are consistent with previously published results, revealing a small but significant temperature sensitivity. Calcium isotope fractionation in tests of N. pachyderma shows a significant variation with temperature, which is not uniform over the total investigated temperature range (−1.6 °C to +10 °C), possibly reflecting the influence of additional controlling factors besides temperature. Controlled dissolution experiments in the laboratory indicate that the Ca-isotope composition of G. sacculifer and N. pachyderma is relatively insensitive to partial dissolution of their tests. Calcium isotope ratios in the planktonic foraminifers G. sacculifer and N. pachyderma (s) reveal a complex Ca isotope fractionation behaviour, which is not yet fully understood. Additional validation studies are crucial to enhance the basic understanding of the calcium isotope systematics in planktic foraminifer shells, and the potential for applying Ca-isotope ratios as proxies for seawater temperature and the oceanic Ca budget.

Nd/Ca ratios in plankton‐towed and core top foraminifera: Confirmation of the water column acquisition of Nd

Planktic foraminifera have been used as recorders of the neodymium (Nd) isotopic composition of seawater, although there is still controversy over the precise provenance of the Nd signal. We present an extensive, multispecific plankton tow Nd/Ca data set from several geographic locations (SE Atlantic, NE Atlantic, Norwegian Sea, and western Mediterranean), together with core top samples from the Mediterranean region. The range of Nd/Ca ratios in plankton‐towed foraminifera, cleaned only of organic material, from all regions (0.01–0.7 μmol/mol), is similar to previously published analyses of sedimentary foraminifera cleaned using both oxidative and reductive steps, with distribution coefficients (Kd) ranging between 4 and 302. For the Mediterranean, where core top and plankton tow data are both available, the range for plankton tows (0.05–0.7 μmol/mol) is essentially identical to that for the core tops (0.1–0.5 μmol/mol). Readsorption of Nd during cleaning is ruled out by the fact that the plankton tow samples underwent only an oxidative cleaning process. We find a relationship between manganese (Mn) and Nd in plankton tow samples that is mirrored by a similar correlation in core top samples. This relationship suggests that Fe‐Mn coatings are of negligible importance to the Nd budgets of foraminifera as the Nd/Mn ratio it implies is over an order of magnitude greater than that seen in other Fe‐Mn oxide phases. Rather, since both plankton tows and core tops present a similar behavior, the Nd/Mn relationship must originate in the upper water column. The data are consistent with the acquisition of Nd and Mn from the water column by binding to organic material and the fact that intratest organic material is shielded from both aggressive cleaning and diagenetic processes. Collectively, the results help to explain two abiding puzzles about Nd in sedimentary planktic foraminifera: their high REE contents and the fact that they record a surface water Nd isotopic signal, regardless of the cleaning procedure used.

Zeroing tests of luminescence sediment dating in the Arctic Ocean: Review and new results from Alaska-margin core tops and central-ocean dirty sea ice

Additional numeric dating techniques are needed for sediments in the Arctic Ocean that span at least the last half million years. Almost all presently suitable geochronometers are relative-dating methods, requiring significant assumptions about sedimentation rates (such as continuous deposition and preservation). An alternative ‘absolute’ geochronometer, luminescence sediment dating, assumes daylight exposure of siliciclastic grains before burial. Early tests and applications of thermoluminescence (TL) techniques in the western Arctic Ocean showed some promise, but trans-ocean application to core tops of more sensitive photon-stimulated-luminescence (PSL) multi-aliquot procedures revealed large regional variations in accuracy. Recent applications of single-aliquot PSL procedures to core tops and dirty sea ice refine our understanding of the limitations and capabilities of PSL dating in this ocean. Dirty sea ice is the source of most of the bottom sediment. Particular procedures for multi-aliquot and single-aliquot PSL dating produce near-zero ages for silt grains from 4 available dirty sea-ice samples. This suggests that TL and PSL age overestimates obtained previously from core-top samples were due mainly to bottom-current reworking (mixing or stripping) of silt grains in abyssal darkness, rather than due to sedimentation of relict-age grains from sea ice. Single-aliquot PSL dating of silt grains from core tops at two different-depth sites at the Alaskan margin reveals large variations in accuracy. This re-emphasizes the critical sensitivity of fine-silt PSL dating accuracy to the effects of bottom-current processes. These recent applications are beginning to provide a more precise knowledge of just where and how PSL methods are likely to be accurate in the Arctic Ocean.

Reassessing Mg/Ca temperature calibrations of Neogloboquadrina pachyderma (sinistral) using paired δ44/40Ca and Mg/Ca measurements

The Mg/Ca temperature calibration of the polar to subpolar planktonic foraminifera Neogloboquadrina pachyderma (sinistral) (sinistral indicates left coiling) was refined by a multiproxy approach combining hydrographic temperature and salinity data with Mg/Ca, δ44/40Ca, and δ18O values from Holocene Nordic seas core top samples. Reliable Mg/Ca‐based temperature estimates are limited to foraminiferal tests that calcified in water masses with temperatures above ∼3°C at habitat depth. In these samples, Mg/Ca and δ44/40Ca values are positively correlated (Mg/Ca (mmol/mol) = 0.77 (±0.22) × δ44/40Ca (‰ SRM 915a) + 0.52 (±0.12); n = 20, R2 = 0.76). Both Mg/Ca‐ and δ44/40Ca‐derived temperatures projected onto their corresponding depth intervals reveal that the “apparent” calcification depth of N. pachyderma (sinistral) averaging the specimens' whole life cycle is bound to an isopycnal layer defined by water densities (σt) between 27.7 and 27.8. This implies that N. pachyderma (sinistral) prefers gradually deeper habitats with increasing sea surface temperatures, thus counterbalancing absolute temperature variations. Consequently, the total temperature range recorded in this foraminiferal species is restricted and only partly reflects environmental changes. On the basis of the new Mg/Ca, δ44/40Ca, and δ18O multiproxy data set, we propose a linear Mg/Ca temperature relation for high‐latitude N. pachyderma (sinistral): Mg/Ca (mmol/mol) = 0.13 (±0.037) T (°C) + 0.35 (±0.17); T &gt; 3°C. In core top samples from polar waters with peak summer temperatures below ∼3°C, the temperature response in the Mg/Ca and δ44/40Ca proxy signal is inversed and poorly correlated. Both Mg/Ca‐ and δ44/40Ca‐derived temperature estimates pretend significantly higher calcification temperatures than maximum summer sea surface temperatures of these water masses.

Calcium isotopic composition of high-latitude proxy carrier &amp;lt;i&amp;gt;Neogloboquadrina pachyderma&amp;lt;/i&amp;gt; (sin.)

Abstract. The accurate reconstruction of sea surface temperature (SST) history in climate-sensitive regions (e.g. tropical and polar oceans) became a challenging task in palaeoceanographic research. Biogenic shell carbonate SST proxies successfully developed for tropical regions often fail in cool water environments. Their major regional shortcomings and the cryptic diversity now found within the major high latitude proxy carrier Neogloboquadrina pachyderma (sin.) highlight an urgent need to explore complementary SST proxies for these cool-water regions. Here we incorporate the genetic component into a calibration study of a new SST proxy for the high latitudes. We found that the calcium isotopic composition (δ44/40Ca) of calcite from genotyped net catches and core-top samples of the planktonic foraminifera Neogloboquadrina pachyderma (sin.) is related to temperature and unaffected by genetic variations. The temperature sensitivity has been found to be 0.17 (±0.02)‰ per 1°C, highlighting its potential for downcore applications in open marine cool-water environments. Our results further indicate that in extreme polar environments, below a critical threshold temperature of 2.0 (±0.5)°C associated with salinities below 33.0 (±0.5)‰, a prominent shift in biomineralization affects the δ44/40Ca of genotyped and core-top N. pachyderma (sin.), becoming insensitive to temperature. These findings highlight the need of more systematic calibration studies on single planktonic foraminiferal species in order to unravel species-specific factors influencing the temperature sensitivity of Ca isotope fractionation and to validate the proxies' applicability.

Stable isotopes of planktonic foraminifera from tropical Atlantic/Caribbean core-tops: Implications for reconstructing upper ocean stratification

δ 18O values of nine tropical–subtropical planktonic foraminiferal species with different preferential habitat depths collected from 62 core–top samples along an east–west transect across the tropical Atlantic/Caribbean were used to test the applicability of interspecific δ 18O gradients for reconstructions of tropical upper ocean stratification. In general, the δ 18O difference (Δ δ 18O) between intermediate- and shallow-dwelling species decreases, and Δ δ 18O between deep and intermediate dwellers increases with increasing thermocline depth towards the west. The statistical significance of regional differences in Δ δ 18O highlights Δ δ 18O between the intermediate dwellers (in particular Globorotalia scitula and Globorotalia tumida) and the shallow dweller Globigerinoides ruber pink, as well as Δ δ 18O between the deep dwellers Globorotalia crassaformis or Globorotalia truncatulinoides dextral and intermediate dwellers as most sensitive to changes in tropical Atlantic thermocline depth. Based on the observed regional variations in interspecific Δ δ 18O, we propose a multispecies stratification index “STRA trop” = ( δ 18O intermediate − δ 18O shallow) / ( δ 18O deep − δ 18O shallow) for the tropical ocean. Statistically significant differences in STRA trop values between the E-Atlantic and the Caribbean suggest that this index may be a useful tool to monitor variations in tropical upper ocean stratification in the geological record.

Intratest oxygen isotope variability in the planktonic foraminifer N. pachyderma: Real vs. apparent vital effects by ion microprobe

Intratest oxygen isotope variations in the planktonic foraminifer Neogloboquadrina pachyderma sinistral (left coiling) from North Atlantic core top and multi-net samples were assessed by ion microprobe analysis from 2 to 6 μm spots with precision and accuracy better than 0.7‰ in δ 18O (2 SD). Within a single foraminiferal test from a core top sample comprising both ontogenetic calcite and outer crust, δ 18O values vary from 0.5‰ to 3.7‰ [PDB], exceeding the range of equilibrium δ 18O in the specimens' habitat by a factor of three. The isotopic difference between the ontogenetic calcite and the crust averages 1.8‰. Neither of the two types of foraminiferal calcite precipitates in equilibrium with ambient seawater. The ontogenetic calcite exhibits a negative vital effect Δ 18O (M-E) ( δ 18O (measured) − δ 18O (equilibrium)) ranging from − 0.5 to − 1‰. The largest negative fractionation is associated to the inner walls of juvenile chambers. In contrast, a positive vital effect of about 0.8‰ was observed in the crust with respect to the highest equilibrium δ 18O values at water depths below 200 m. Hence two vital effects that are opposite in sign are effective within a single foraminiferal test, indicating that ‘whole test’ values of this species are highly sensitive to the degree of encrustation and amplify or attenuate environmental signals. The negative vital effect of the ontogenetic calcite was verified by ion microprobe analysis of four nonencrusted net-sampled specimens reflecting three different depth intervals. Intra-ontogenetic oxygen isotope ratios in these juvenile tests range from 0.6 to 3.0‰ and exhibit a negative vital effect even larger than that observed in core top samples. Based on these data, the large range of ‘apparent’ vital effects reported for this foraminiferal species can be assessed by a mass balance calculation, assuming that the degree of encrustation is variable.

Mg/Ca in the benthic foraminifera Cibicidoides wuellerstorfi and Cibicidoides mundulus: Temperature versus carbonate ion saturation

Magnesium/calcium (Mg/Ca) ratios in two widely used benthic foraminiferal species, Cibicidoides wuellerstorfi and Cibicidoides mundulus ( Cibicidoides kullenbergi), picked from global ocean core-tops and from six cores in the North Atlantic Ocean have been measured to investigate Mg/Ca variability, cleaning effect, and influences from bottom water temperature (BWT) and deep water carbonate ion saturation (Δ[CO 3 2−]). Replicate measurements of the two species reveal Mg/Ca variability of 0.07 ± 0.07 mmol/mol. Compared with Mg/Ca in samples cleaned by oxidative cleaning, Mg/Ca ratios are significantly lowered, by 0.10 ± 0.09 mmol/mol, in samples cleaned by reductive cleaning, likely due to preferential leaching during the reductive cleaning step. The cleaning influence is large relative to glacial–interglacial Mg/Ca changes (0.05–0.31 mmol/mol by this study). Regression of core-top data from a wide range of locations show that C. wuellerstorfi Mg/Ca is strongly affected by deep water Δ[CO 3 2−] with a sensitivity of ~ 0.009 mmol/mol per µmol/kg. When BWT is included in regressions, the correlation is slightly more significant with a weak BWT sensitivity on Mg/Ca of between 0.03 ± 0.01 and 0.07 ± 0.02 mmol/mol per °C. Because Δ[CO 3 2−] and BWT co-vary for most core-top samples, an alternate method of separating the two effects was made by comparing glacial to interglacial changes in Mg/Ca from a depth transect of North Atlantic cores where the BWT and carbonate ion histories since the last glacial period have been well constrained. Results from this transect suggest that the BWT effect is < 0.03 mmol/mol per °C, weaker than inferred from the core-top study. Therefore, core-top and down core data suggest that C. wuellerstorfi Mg/Ca is insensitive to BWT changes. The weak BWT effect, when compared with natural variability, complicates the use of Mg/Ca in this species to reconstruct past BWT. Taking all measured samples from various environments into account, C. mundulus Mg/Ca ratios show no correlation with BWT or deep water Δ[CO 3 2−].

Quantitative summer-temperature reconstructions for the last 2000 years based on pollen-stratigraphical data from northern Fennoscandia

Quantitative reconstructions of mean July temperatures (T jul) based on new and previously published pollen-stratigraphical data covering the last 2000 years from 11 lakes in northern Fennoscandia and the Kola Peninsula are presented. T jul values are based on a previously published pollen-climate transfer function for the region with a root-mean-square error of prediction (RMSEP) of 0.99°C. The most obvious trend in the inferred temperatures from all sites is the general decrease in T jul during the last 2000 years. Pollen-inferred T jul values on average 0.18 ± 0.56°C (n = 91) higher than present (where “present” refers to the last three decades based on pollen-inferred T jul in core-top samples) are indicated between 0 and 1100 AD (2000–850 cal year BP), and temperatures −0.2 ± 0.47°C (n = 78) below present are inferred between 1100 and 1900 AD (850–50 cal year BP). No consistent temperature peak is observed during the ‘Medieval Warm Period’, ca. 900–1200 AD (1100–750 cal year BP), but the cooler period between 1100 and 1900 AD (850–50 cal year BP) corresponds in general with the ‘Little Ice Age’ (LIA). Consistently with independent stable isotopic data, the composite pollen-based record suggests that the coldest periods of the LIA date to 1500–1600 AD (450–350 cal year BP) and 1800–1850 AD (150–100 cal year BP). An abrupt warming occurred at about 1900 AD and the twentieth century is the warmest century since about 1000 AD (950 cal year BP).

Seasonal and interannual variability of the planktic foraminiferal flux in the vicinity of the Azores Current

Planktic foraminiferal (PF) flux and faunal composition from three sediment trap time series of 2002–2004 in the northeastern Atlantic show pronounced year-to-year variations despite similar sea surface temperature (SST). The averaged fauna of the in 2002/2003 is dominated by the species Globigerinita glutinata, whereas in 2003/2004 the averaged fauna is dominated by Globigerinoides ruber. We show that PF species respond primarily to productivity, triggered by the seasonal dynamics of vertical stratification of the upper water column. Multivariate statistical analysis reveals three distinct species groups, linked to bulk particle flux, to chlorophyll concentrations and to summer/fall oligotrophy with high SST and stratification. We speculate that the distinct nutrition strategies of strictly asymbiontic, facultatively symbiontic, and symbiontic species may play a key role in explaining their abundances and temporal succession. Advection of water masses within the Azores Current and species expatriation result in a highly diverse PF assemblage. The Azores Frontal Zone may have influenced the trap site in 2002, indicated by subsurface water cooling, by highest PF flux and high flux of the deep-dwelling species Globorotalia scitula. Similarity analyses with core top samples from the global ocean including 746 sites from the Atlantic suggest that the trap faunas have only poor analogs in the surface sediments. These differences have to be taken into account when estimating past oceanic properties from sediment PF data in the eastern subtropical North Atlantic.