The use of planktonic foraminifera transfer function and Hill sigmoidal fit to reconstruct upper ocean thermal stratification

The ongoing anthropogenic‐induced warming assessment requires a robust background from regional sea surface temperature (SST) reconstructions. Planktonic foraminifera have yielded valuable insights into late Quaternary SST dynamics, but the techniques to estimate SST from fossil assemblages have only rarely been used in very recent sedimentary records (the last 2,000 years). Here we use two transfer function methods, modern analog technique and artificial neural networks, to reconstruct SST variability in two cores from the Central Mediterranean Sea that span the last five centuries. Both cores show similar and considerable changes in the planktonic foraminifera assemblages. However, the inferred mean annual SSTs only varied in a narrow range, in agreement with instrumental data that go back to 1850 CE. Our reconstructions extend this time frame and indicate that SST variability did not exceed 1.5 °C over the past three centuries. Rather than temperature, we suggest that the changes in the assemblages reflect switches between sea surface winter/spring productivity and a deep winter mixed layer, due to the atmosphere/ocean interplay that governs different productivity modes in neighboring mesoscale gyres.

\n During the last glacial and deglacial periods, the Earth experienced several abrupt millennial-scale climate change events, named Heinrich Stadials (HS) and Dansgaard-Oeschger events. The HS in particular are commonly attributed to reductions in the strength of the Atlantic meridional overturning circulation (AMOC). Given the marked AMOC influence over global climate and the possibility of the AMOC to reduce its strength in the future due to ongoing climate change, the study of HS became a topic of key importance. Here we investigate the effects of last glacial and deglacial HS to eastern (E) South American hydroclimate as well as western South Atlantic oceanography. To do so, we studied marine sediment core M125-95-3 collected from the western tropical South Atlantic mid-depth (10.94°S, 36.20°W, 1897 m water depth), near the mouth of the São Francisco River (i.e., off E South America), spanning the last ca. 70,000 years. We produced radiocarbon ages from planktonic foraminifera, X-ray fluorescence analyses from bulk sediment samples, stable oxygen and carbon isotopic analyses from planktonic and benthic foraminifera, and Mg/Ca analyses from planktonic foraminifera. We conclude that the last glacial and deglacial HS were marked by positive precipitation anomalies over the São Francisco River drainage basin, and that this was the southernmost drainage basin from the South American Atlantic seaboard that experienced substantial increases in precipitation. We propose a new mechanism for explaining tropical South America HS positive precipitation anomalies. This mechanism involves austral summer precipitation increases only over E South America while the rest of tropical South America experienced precipitation increases during the winter, challenging the widely held assumption of a strengthened monsoon during HS. During the same abrupt events, the mid-depth western tropical South Atlantic experienced decreases in d13C and increases in sulfur (unprecedentedly used as a proxy for abrupt millennial-scale changes in bottom water ventilation) that we attributed to an increased Northern Component Water (NCW) residence time and to the accumulation of respired carbon at mid-depths. We also suggest that the negative d13C excursions progressively increase along the NCW southwards pathway, reaching its maximum in the western tropical South Atlantic from where the signal dissipates/dilutes by mixing with Southern Component Water. Regarding the upper water column, the western tropical South Atlantic surface waters were warmer and saltier during HS. Data from the Agulhas Leakage region also recorded similar features, however, with larger positive excursions. We conclude that the heat and salt imported from the Indian Ocean during HS were only partially transferred to the western tropical South Atlantic. Thus, Indian Ocean salt that eventually reached the high latitudes of the North Atlantic helping on the recovering of the AMOC was most probably transported mainly within the thermocline. Finally, the data shown herein indicate that past events of weak AMOC profoundly affected South American hydroclimate and western South Atlantic oceanography\n

Sea surface temperatures were reconstructed over the last 30,000 years from alkenone paleothermometry (SST‐alk) and planktonic foraminifera assemblages using the Modern Analog Technique (MAT) (SST‐foram) along two cores of the Mediterranean Sea: MD84‐632 (Levantine basin) and MD04‐2797 (Siculo‐Tunisian Strait). Oxygen isotope of planktonic foraminifera G. bulloides for core MD04‐2797 and G. ruber for core MD84‐632 were also determined. SST‐alk in the Levantine basin indicate colder values at the Last Glacial Maximum (LGM) (∼14°C) than earlier established from MAT, and a cooling amplitude of 6°–7°C, comparable to the central Mediterranean Sea. Climatic events such as the Younger Dryas (YD) and Heinrich events 1 and 2 (H1 and H2) were times of significant cooling in the two cores. In the Eastern basin, values of local seawater oxygen isotope, δw, indicate relatively saltier waters during the LGM and deglaciation than today, with increasing δw values (higher salinity) in the Eastern basin and decreasing ones (lower salinity) in the central Mediterranean Sea, during cold stadials. The observed alterations of surface water properties (T and δw) in the central and eastern Mediterranean at the LGM are consistent with model experiments showing slightly lower evaporation in the Mediterranean than today, except for the Eastern basin.

Planktonic foraminifera census data have been widely used to reconstruct changes in ocean ecosystems as well as ocean chemistry and circulation through calibration functions. Here we analyse new core-top census data from 22 sites in the western South Atlantic, improving the geographical coverage and spatial resolution of the environmental gradients from the region covered by the Brazil–Malvinas Confluence (˜38°S–53°W). We combine them with previous data to provide an up-to-date compilation of the western South Atlantic planktonic foraminifera–calibration data set. We study the relationship between the assemblages present in the core-top samples and the most dynamic environmental variables from the region to establish the environmental variable(s) more resolved by the assemblages. Therefore, we develop and assess a new calibration function using the data set and testing several statistical models at different water depths. Our results reveal that the distribution and abundance of the species in the Brazil–Malvinas Confluence region mainly reflect the influence of two environmental variables: the mixed layer temperature and mixed layer depth (57% of total variance). The most precise reconstructions were obtained when using sub-surface temperatures between 40 and 50 m water depth. The application of the calibration function to a Holocene sediment core at ˜37°S–53°W revealed mean annual sub-surface temperature reconstructions between 8°C and 11°C, confirming the northward displacement of the Brazil–Malvinas Confluence during the onset of the Holocene and suggesting a major influence of the Malvinas Current during the entire Holocene at the studied site.

Sediment trap and deep sea coretop sediments as tracers of recent changes in planktonic foraminifera assemblages in the southeastern ultra-oligotrophic Levantine Basin

We have investigated glacial‐interglacial differences in sea surface temperature (SST) near Hawaii using two relatively high deposition rate, shallow‐water piston cores collected near Oahu, Hawaii. Modern hydrographic data show that local surface water temperatures are broadly consistent with the regional pattern of SSTs in the southern subtropical North Pacific. Past SSTs were estimated on the basis of three independently measured parameters: (1) U37k′ values of alkenones, (2) δ18O of Globigerinoides ruber, and (3) assemblages of planktonic foraminifera using the modern analog technique (MAT). The two cores yield similar SST records, and if differences in the ecology of foraminifera and coccolithophores are considered, the three different approaches to estimating SSTs yield consistent results. U37k′‐based temperatures, which may represent winter values at this location, were ∼2.5°C colder during the Last Glacial Maximum than today, which is consistent with the February MAT estimates. The δ18O‐based temperature estimates, likely biased toward summer temperatures, indicate that the glacial SSTs were at least 1°C cooler than today, which is comparable to the results of MAT August estimates.

Abstract. The species composition of many groups of marine plankton appears well predicted by sea surface temperature (SST). Consequently, fossil plankton assemblages have been widely used to reconstruct past SST. Most applications of this approach make use of the highest possible taxonomic resolution. However, not all species are sensitive to temperature, and their distribution may be governed by other parameters. There are thus reasons to question the merit of including information about all species, both for transfer function performance and for its effect on reconstructions. Here we investigate the effect of species selection on planktonic foraminifera transfer functions. We assess species importance for transfer function models using a random forest technique and evaluate the performance of models with an increasing number of species. Irrespective of using models that use the entire training set (weighted averaging) or models that use only a subset of the training set (modern analogue technique), we find that the majority of foraminifera species does not carry useful information for temperature reconstruction. Less than one-third of the species in the training set is required to provide a temperature estimate with a prediction error comparable to a transfer function that uses all species in the training set. However, species selection matters for paleotemperature estimates. We find that transfer function models with a different number of species but with the same error may yield different reconstructions of sea surface temperature when applied to the same fossil assemblages. This ambiguity in the reconstructions implies that fossil assemblage change reflects a combination of temperature and other environmental factors. The contribution of the additional factors is site and time specific, indicating ecological and geological complexity in the formation of the sedimentary assemblages. The possibility of obtaining multiple different reconstructions from a single sediment record presents a previously unrecognized source of uncertainty for sea surface temperature estimates based on planktonic foraminifera assemblages. This uncertainty can be evaluated by determining the sensitivity of the reconstructions to species pruning.

To better understand production, succession, excystment and transport of dinoflagellate cysts (dinocysts) and planktonic foraminifera in the upper water column, we investigated their fluxes during a 7-day survey in the active upwelling off Cape Blanc (NW Africa) in November 2018 with drifting traps at 100 m, 200 m and 400 m water depth. The survey covered a change from active upwelling to stratified conditions. Highest production of organic dinocysts and planktonic foraminifera was observed during active upwelling conditions and decreased drastically towards the end of the survey. Calcareous dinocysts appeared later during upwelling relaxation. Cytoplasm-bearing (full) dinocysts and foraminifera were produced in the water column above the traps (<100 m depth). Some of the empty dinocysts were resuspended, implying that sediments below the survey site contain both local and allochthonous cyst assemblages. This is the first demonstration that excystment in the upper water column is species-specific. Brigantedinium excysted in the upper water column before reaching deeper depths, whereas no upper water column excystment was observed for the other dinoflagellate species. Dinoflagellate and planktonic foraminifera associations showed a clear succession. During active upwelling, Echinidinium zonneveldiae, Brigantedinium spp., other peridinioids, Echinidinium spp., cysts of Pentapharsodinium dalei, ‘other photosynthetic organic-walled dinocysts‘, Neogloboquadrina incompta and Globigerinella calida were collected. During upwelling relaxation, Lingulodinium machaerophorum was produced; and under stratified conditions Gymnodiniaceae cysts (G. microreticulatum, G. catenatum) and the foraminifera Globigerina bulloides and Orbulina universa were sampled. Apart from enhancing knowledge of these species, our observations allow more detailed reconstructions of upwelling history in the Cape Blanc region based on sedimentary archives using fossilized dinoflagellate and planktonic foraminifera assemblages.

Global cooling in marine climates and local tectonic events in Southwest Japan at the Plio–Pleistocene boundary

Modern analog technique, often abbreviated as MAT, is a popular type of transfer function used in paleoceanography to reconstruct past ocean properties from the composition of fossil assemblages extracted from deep-sea sediments. In the vast majority of cases, MAT is applied to reconstruct past sea-surface temperature from remains of marine unicellular microplankton such as planktonic foraminifera, radiolaria, diatoms, and dinoflagellates. The method has been introduced to paleoceanography by Hutson (1980). Mathematically, it is akin to k-nearest neighbor regression. Its procedure is computationally and conceptually very simple. First, a similarity coefficient is calculated between the composition of a fossil assemblage in a geological sample and the compositions of corresponding assemblages in a “calibration dataset” of surface-sediment samples. Then, the surface-sediment samples are sorted by their similarity coefficient, and a subset of those with most similar assemblages, termed the “nearest” or “best modern analogs,” is used in the second step to calculate an estimate of the desired ocean property. The estimate represents a mean of the presentday values of the ocean property at the site of the deposition of the best modern analogs, typically weighted by the respective similarity coefficient of each sample. Like all paleoceanographic transfer functions, MAT relies on the assumption that the reconstructed ocean property is a statistically important determinant of the assemblage composition of the studied organisms. It also assumes that the fossil assemblage and its covariance with the ocean property are represented in the calibration dataset. If this is not the case, the fossil assemblage is said to represent a no-analog condition, which may render the reconstruction of past ocean properties invalid. MAT is a strict interpolator; it does not attempt to extract a general relationship between assemblage composition and environmental factors. Its applicability is therefore contingent on appropriate coverage of the environmental gradient and of the associated assemblage compositions. Because of no-analog conditions due to evolution of niche requirements of the constituent species, assemblage-based transfer functions are typically limited in their applicability to late Quaternary sediments. Sea-surface temperature reconstructions via MATcan be obtained to a precision of 1 C, and the variance among the SST values of the best analogs can be used to assess the uncertainty of a fossil estimate.

We present a data set of 738 planktonic foraminiferal species counts from sediment surface samples of the eastern North Atlantic and the South Atlantic between 87°N and 40°S, 35°E and 60°W including published Climate: Long‐Range Investigation, Mapping, and Prediction (CLIMAP) data. These species counts are linked to Levitus's [1982] modern water temperature data for the four caloric seasons, four depth ranges (0, 30, 50, and 75 m), and the combined means of those depth ranges. The relation between planktonic foraminiferal assemblages and sea surface temperature (SST) data is estimated using the newly developed SIMMAX technique, which is an acronym for a modern analog technique (MAT) with a similarity index, based on (1) the scalar product of the normalized faunal percentages and (2) a weighting procedure of the modern analog's SSTs according to the inverse geographical distances of the most similar samples. Compared to the classical CLIMAP transfer technique and conventional MAT techniques, SIMMAX provides a more confident reconstruction of paleo‐SSTs (correlation coefficient is 0.994 for the caloric winter and 0.993 for caloric summer). The standard deviation of the residuals is 0.90°C for caloric winter and 0.96°C for caloric summer at 0‐m water depth. The SST estimates reach optimum stability (standard deviation of the residuals is 0.88°C) at the average 0– to 75‐m water depth. Our extensive database provides SST estimates over a range of −1.4 to 27.2°C for caloric winter and 0.4 to 28.6°C for caloric summer, allowing SST estimates which are especially valuable for the high‐latitude Atlantic during glacial times.An electronic supplement of this material may be obtained on adiskette or Anonymous FTP from KOSMOS.AGU.ORG. (LOGIN toAGU's FTP account using ANONYMOUS as the username and GUESTas the password. Go to the right directory by typing CD APPEND. TypeLS to see what files are available. Type GET and the name of the file toget it. Finally type EXIT to leave the system.) (Paper 95PA01743,SIMMAX: A modern analog technique to deduce Atlantic sea surfacetemperatures from planktonic foraminifera in deep‐sea sediments, UwePflaumann, Josette Duprat, Claude Pujol, and Laurent D. Labeyrie).Diskette may be ordered from American Geophysical Union, 2000Florida Avenue, N.W., Washington, DC 20009; Payment mustaccompany order.

Surface water conditions in the Northern Benguela Region (SE Atlantic) during the last 450 ky reconstructed from assemblages of planktonic foraminifera

Abstract. We assessed sediment coring, data acquisition, and publications from the North Pacific (north of 30° N) from 1951 to 2016. There are 2134 sediment cores collected by American, French, Japanese, Russian, and international research vessels across the North Pacific (including the Pacific subarctic gyre, Alaskan gyre, Japan margin, and California margin; 1391 cores), the Sea of Okhotsk (271 cores), the Bering Sea (123 cores), and the Sea of Japan (349 cores) reported here. All existing metadata associated with these sediment cores are documented here, including coring date, location, core number, cruise number, water depth, vessel metadata, and coring technology. North Pacific sediment core age models are built with isotope stratigraphy, radiocarbon dating, magnetostratigraphy, biostratigraphy, tephrochronology, % opal, color, and lithological proxies. Here, we evaluate the iterative generation of each published age model and provide comprehensive documentation of the dating techniques used, along with sedimentation rates and age ranges. We categorized cores according to the availability of a variety of proxy evidence, including biological (e.g., benthic and planktonic foraminifera assemblages), geochemical (e.g., major trace element concentrations), isotopic (e.g., bulk sediment nitrogen, oxygen, and carbon isotopes), and stratigraphic (e.g., preserved laminations) proxies. This database is a unique resource to the paleoceanographic and paleoclimate communities and provides cohesive accessibility to sedimentary sequences, age model development, and proxies. The data set is publicly available through PANGAEA at https://doi.org/10.1594/PANGAEA.875998.

Ocean-floor sediment samples collected up to 150 years ago represent an important historical archive to benchmark global changes in the seafloor environment, such as species' range shifts and invasions and pollution trends. Such benchmarking requires that the historical sediment samples represent the state of the environment at - or shortly before the time of collection. However, early oceanographic expeditions sampled the ocean floor using devices like the sounding tube or a dredge, which potentially disturb the sediment surface and recover a mix of Holocene (surface) and deeper, Pleistocene sediments. Here we use climate-sensitive microfossils as a fast biometric method to assess if historical seafloor samples contain a mixture of modern and glacial sediments. Our assessment is based on comparing the composition of planktonic foraminifera (PF) assemblages in historical samples with Holocene and Last Glacial Maximum (LGM) global reference datasets. We show that eight out of the nine historical samples contain PF assemblages more similar to the Holocene than to the LGM PF assemblages, but the comparisons are only significant when there is a high local species' temporal turnover (from the LGM to the Holocene). When analysing temporal turnover globally, we show that upwelling and temperate regions had greatest species turnover, which are areas where our methodology would be most diagnostic. Our results suggest that sediment samples from historical collections can provide a baseline of the state of marine ecosystems in the late 19th century, and thus be used to assess ocean global change trends.

Calcareous microfossils are widely used by paleoceanographers to investigate past sea-surface hydrology. Among these microfossils, planktonic foraminifera are probably the most extensively used tool (e.g. [1] for a review), as they are easy to extract from the sediment and can also be used for coupled geochemical (e.g; δ18O, δ13C, Mg/Ca) and paleo-ecological investigations. Planktonic foraminifera are marine protists, which build a calcareous shell made of several chambers which reflect in their chemistry the properties of the ambient water-masses. Planktonic foraminifera are known to thrive in various habitats, distributed not only along a latitudinal gradient, but also along different water-depth intervals within surface waters (0-1000 m). Regarding their biogeographical distribution, planktonic foraminifera assemblages therefore mirror different water-masses properties, such as temperature, salinity and nutrient content of the surface water in which they live. The investigation of the specific composition of a fossil assemblage (relative abundances) is therefore a way to empirically obtain (paleo)information on past variations of sea-surface hydrological parameters. This paper focuses on the planktonic foraminifera record from the Arctic domain. This polar region records peculiar sea-surface conditions, with the influence of nearly perennial sea-ice cover development. This has strong impact on living foraminifera populations and on the preservation of their shells in the underlying sediments.