Marine sediments from southeastern Brazilian continental shelf: A 1200 year record of upwelling productivity

A sediment core from the Cabo Frio coastal shelf (?23.19 S, ?41.8 W; 117 m depth), was analyzed for TOC, C/N ratio, organic petrography and planktonic foraminiferal content to evaluate variations in local productivity caused by changes in upwelling intensity and its relation to regional and global climatic variations during the last millennium. The Cabo Frio core recorded the last 1200 years of sedimentation, with rates varying from 0.11 to 0.32 mm yr^?1. Foraminiferal and organic geochemical analyses indicate the occurrence of three distinct periods of productivity. From 850 AD until 1070 AD, foraminifera fluxes consisting primarily of Turborotalita quinqueloba indicate stronger South Atlantic Central Water (SACW) transport onto the shelf, which induced high biological productivity that was also recorded by high TOC and marine palynomorphs content and a low C/N atomic ratio. This period coincided with a northward displacement of the atmospheric Intertropical Convergence Zone (ITCZ) and South Atlantic High (SAH) systems driven by positive temperature anomalies in the North Atlantic Ocean during the Medieval Climate Anomaly (MCA). From 1070 until 1500 AD, low TOC flux and planktonic foraminifera fluxes and high C/N atomic ratios suggest a reduction in marine productivity, probably driven by reduced transport of SACW associated with the southward displacement of the SAH and weakening of northeasterly winds. The period between 1500 and 1830 AD, which corresponds to the Little Ice Age, is marked by increased fluxes of planktonic foraminifera, principally of Globigerina bulloides and Globigerinita glutinata. These species mark an increase in productivity linked to SACW upwelling, supported by the enhancement of northeasterly winds and southward displacement of the ITCZ and SAH.     

Sea surface temperature changes during May and August in the western Arabian Sea over the last 22 kyr: Implications as to shifting of the upwelling season

In the western Arabian Sea (WAS), the highest seasonal sea surface temperature (SST) difference presently occurs between May and August. In order to gain an understanding on how monsoonal upwelling modulates the SST difference between these two months, we have computed SST for the months of May and August based on census counts of planktonic foraminifers by using the artificial neural network (ANN) technique. The SST difference between May and August exhibits three distinct phases: i) a moderate SST difference in the late Holocene (0–3.5 ka) is attributable to intense upwelling during August, ii) a minimum SST difference from 4 to 12 ka is due to weak upwelling during the month of August, and iii) the highest SST difference during the last glacial interval (19 to 22 ka) with high Globigerina bulloides % could have been caused by the occurrence of a prolonged upwelling season (from May through July) and maximum difference in the incoming solar radiation between May and August. Overall, variations in the SST difference between May and August show that the timing of intense upwelling in the Western Arabian Sea over the last 22 kyr has been variable over the months of June, July and August.     

Foraminifera and coccolithophorid assemblage changes in the Panama Basin during the last deglaciation: Response to sea-surface productivity induced by a transient climate change

The responses of community assemblages of planktonic and benthonic foraminifera and coccolithophorids to transient climate change are explored for the uppermost 2 m of cores ODP677B (1.2°N; 83.74°W, 3461 m) and TR163-38 (1.34°S; 81.58°W, 2200 m), for the last ∼ 40 ka. Results suggest that the deglaciation interval was a time of increased productivity and a major reorganization of planktonic trophic webs. The succession in dominance between the planktonic foraminifera species Globorotalia inflata, Globigerina bulloides, and Neogloboquadrina pachyderma denote four periods of oceanographic change: (1) advection (24-20 ka), (2) strong upwelling (20-15 ka), (3) weak upwelling (14-8 ka) and (4) oligotrophy (8 ka to present). Strong upwelling for the deglaciation interval is supported by the low Florisphaera profunda/other coccolithophorids ratio and the high percentage abundance of Gephyrocapsa oceanica. Benthonic foraminifera assemblage changes are different in both cores and suggest significant regional variations in surface productivity and/or oxygen content at the seafloor, and a decoupling between surface productivity and export production to the seafloor. This decoupling is evidenced by the inverse relationship between the percentage abundance of infaunal benthonic foraminifera and the percentage abundance of N. pachyderma. The terrigenous input of the Colombian Pacific rivers, particularly the San Juan River, is suggested as a possible mechanism. Finally, the Globorotalia cultrata/Neogloboquadrina dutertrei ratio is used to reconstruct the past influence of the Costa Rica Dome-Panama Bight and cold tongue upwelling systems in the Panama Basin. A northern influence is suggested for the late Holocene (after 5 ka) and the last glacial (before 20 ka), whereas a southern influence is suggested for the 20-5 ka interval. There is a correspondence between our reconstructed northern and southern influences and previously proposed positions of the Intertropical Convergence Zone (ITCZ) after the Last Glacial Maximum (LGM).     

Environmental and climatic changes in the central Mediterranean Sea (Siculo–Tunisian Strait) during the last 30 ka based on dinoflagellate cyst and planktonic foraminifera assemblages

A high resolution micropalaeontological study of the core MD 04-2797 CQ recovered in the Sicilian–Tunisian Strait provides insights into the paleoclimatic history of the Mediterranean Sea at the transition between the western and eastern basin over the last 30 ka. Using the analysis of dinoflagellate cyst and planktonic foraminiferal assemblages, we reconstruct the paleoenvironmental changes that took place in this region. High abundances of cold temperate dinocyst species (Nematosphaeropsis labyrinthus, Spiniferites elongatus, Bitectatodinium tepikiense) and the polar planktonic foraminifera Neogloboquadrina pachyderma (left coiling) reveal three major cooling events synchronous with North Atlantic Henrich events 1 and 2 (H1 and H2) and the European and North Atlantic Younger Dryas event. During the Holocene, the presence of warm dinocyst species (Spiniferites mirabilis and Impagidinium aculeatum) and planktonic foraminifera (Globorotalia inflata and Globigerinoides ruber), reflects a significant increase of sea surface temperatures in the western Mediterranean basin, but a full warming was not recorded until 1500 years after the onset of the Holocene. Moreover, our results show that the Holocene was interrupted by at least four brief cooling events at ~ 9.2 ka, ~ 8 ka, ~ 7 ka and ~ 2.2 ka cal. BP, which may be correlated to climatic events recorded in Greenland ice cores and in the Atlantic Ocean.     

High-resolution sea surface temperature and salinity dynamics in the northern Okinawa Trough over the last 24 kyr

Based on the high-resolution Mg/Ca and oxygen isotope ratio (δ¹⁸O) of planktonic foraminifera Globigerinoides ruber, sea surface temperature (SST) and salinity (SSS) were reconstructed in Core PC-1 from the northern Okinawa Trough during the last 24 kyr. From the last glacial maximum to Holocene, SST varied from 20.2°C to 24.6°C. Millennial-scale climatic events of the SST during the last glacial, such as the Heinrich events, Bølling-Allerød warming, and Younger Dryas, have been identified, which are synchronous with the climate changes in the North Atlantic, suggesting a teleconnection between the northwestern Pacific and the North Atlantic. During the last 24 kyr, the SSS variation can be divided into three parts: (1) during 21–15.5 ka, there was a significantly low SSS; (2) during 15.5–11.7 ka, the SSS increased obviously; (3) since 11.7 ka, the SSS is relatively stable. In order to discuss the main factors influencing the SSS in the northern Okinawa Trough, the relative abundance of Pulleniatina obliquiloculata and the oxygen isotope difference between the northern and middle Okinawa Trough (Δδ¹⁸O_sw) have been used to indicate the intensity of the Kuroshio Current and Changjiang freshwater discharge, respectively. The Δδ¹⁸O_sw result shows that there is a large amount of Changjiang freshwater emptied into the northern Okinawa Trough during 18–15.5 ka, which is caused by the subtropical monsoon rain band lingering in the lower reaches of the Changjiang River drainage, supporting the Jet Transition Hypothesis. As the Kuroshio Current strengthened since 15.5 ka as indicated by the increased relative abundance of P. obliquiloculata, the variation of Kuroshio Current became the more important contribution to the SSS in the study region. The results of our study indicate that the key factor influencing the SSS in the northern Okinawa Trough is variable during the last 24 kyr.     

Fluxes and isotopic composition of planktonic foraminifera off Hainan Island,northern South China Sea: Implications for paleoceanographic studies

Planktonic foraminifera collected from a sediment trap deployed off Hainan in the northwestern South China Sea (SCS-NW) between July 2012 and April 2013 were studied to evaluate their seasonal variability and ecology as well as to infer the factors controlling their shell fluxes. The total planktonic foraminifera flux, as well as the fluxes of the dominant species (Globigerinoides ruber, Globigerinoides sacculifer and Neogloboquadrina dutertrei), showed three distinct maxima during SW-monsoon in August 2012, the SW-NE intermonsoon in October 2012 and the NE-monsoon in December 2012–February 2013. These periods were characterized by upwelling, aerosol fallout, and intense wind mixing, respectively, from which the foraminiferal assemblages benefitted, as indicated by the close correlation between wind speed, sea surface temperature (SST), chlorophyll a concentration (Chl-a), δ¹⁸O of G. ruber and the shell fluxes. The correlation also suggests that temperature and food availability might have been the primary drivers of the observed changes in foraminiferal abundance. The offset between the SST deduced from flux-weighted of G. ruber δ¹⁸O and annual mean SST is only ∼0.3 °C, much lower than ∼5.2 °C between the summer and winter temperature, indicating a balanced seasonality bias in the shell flux. The linear regression between the satellite-derived sea surface temperature and G. ruber δ¹⁸O reveals the strong potential of this species, at least in the studied region, as an ecological indicator for past oceanic environments.     

Sea surface environment inferred from planktonic foraminifera in the southern South China Sea since the last glacial period

Detailed analyses of planktonic foraminifera at Site 17964 from the southern South China Sea (SCS) disclose that warm-water species have a higher percentage during the Holocene, while temperate-water species have a higher content for the last glacial period. Therefore, the sea surface temperature (SST) is a main factor that affects the foraminiferal assemblage at this site. A remarkable faunal variation at Site 17964 is recognized for Pulleniatina obliquiloculata over the last glacial–interglacial periods: higher P. obliquiloculata content during the glacial period and abrupt drop at the beginning of Termination I (16.5–15 kyr B.P.). The characteristic P. obliquiloculata variation can be correlated with other sites in the southern SCS and thus can be adopted as a stratigraphic tool in the region. A detailed analysis of Orbulina universa shell morphometrics at Site 17964 shows the test size from 0.83 to 1.45 mm and the shell porosity up to 36.7%, much larger than those in the Indian and Atlantic Oceans, which indicates a warmer and less saline surface water in the equatorial–tropical western Pacific. The diameter and shell porosity of O. universa increased from the last glacial to the Holocene, corresponding to the increase of SST recorded by the U^k₃₇ alkenone index. A higher correlation coefficient (89%) between the O. universa test size and SST implies that intraspecific O. universa test size be used as an index of the sea surface temperature in the South China Sea.     

Magnesium and strontium compositions of recent symbiont-bearing benthic foraminifera

Minor and trace elements in foraminiferal carbonates are potential paleo-proxies of climate, nutrient and seawater composition. There are very few reports of trace element composition of symbiont-bearing, larger foraminifera that are known to be important constituents of shallow-marine, modern and ancient carbonates. In this paper we examine the range of variation in Mg and Sr content of Recent species of these foraminifera from a lagoon of Lakshadweep Atoll (Indian Ocean) and Akajima Islands, Japan. Two hyaline species, Amphistegina lessonii and Neorotalia calcar,and two porcellaneous species, Amphisorus hemprichii and Marginopora vertebralis were collected live from Lakshadweep islands. Mg / Ca in these foraminifera is of an order of magnitude higher than the values reported for planktonic and symbiont-free benthic foraminifera. The Sr / Ca values are, however, comparable with the reported values in other foraminiferal taxa and they are found to vary within a narrow range. Electron-probe micro-analysis of three symbiont-bearing benthic species indicates spatial heterogeneity of high orders in Mg / Ca composition in all the species. The annual variation in temperature and pH of the lagoon water cannot explain the observed amplitude of the compositional variation. The photosynthesis and respiration of the symbionts and host foraminifera are possibly the major cause of compositional heterogeneity in individual tests, as has also been recently postulated for symbiont-bearing planktonic foraminiferal species. It highlights the need to isolate biological factors and necessitates species-specific paleotemperature scale in paleoclimatic analysis. We also analyzed δ¹⁸O, δ¹³C, Ca, Mg and Sr in carefully dissected chambers of a reef-dwelling, porcellaneous benthic foraminifer, Marginopora kudakajimaensis, collected live in four seasons. A moderate positive correlation is observed between Mg / Ca and temperature. However, large inter- and intra-test variation in Mg limits the precision of Mg / Ca as palaeotemperature proxy. The Sr / Ca of the test calcite is unrelated to temperature of the sea water. The δ¹³C of M. kudakajimaensis does not correlate with δ¹⁸O, Mg / Ca or Sr / Ca.     

Sea-surface temperature for left-coiling Neogloboquadrina populations inhabiting the westernmost Mediterranean in the middle Pleistocene and the Pleistocene-Pliocene transition

Using the modern analog methodology applied to planktonic foraminifers, we analyze the relation between the frequency of the coiling type in Neogloboquadrina populations and the sea-surface temperatures (SST) during the middle Pleistocene and the Pleistocene-Pliocene transition in the Alboran Sea (westernmost Mediterranean), close to the Atlantic connection. The results reveal that the present-day positive correlation between the two variables (r = 0.649) is maintained even with a higher coefficient (r = 0.783) in the middle Pleistocene but falls slightly (r = 0.517) in the Pleistocene-Pliocene transition due mainly to a dispersal of the temperatures for the samples bearing predominantly left-coiling Neogloboquadrina. The temperature used as a reference for the coiling change resulted 5-6 °C above its North Atlantic present-day reciprocal, but these thermal differences could be caused by: i) a slight overestimate (1–2 °C) due to the lumping of all neogloboquadrinids into a single variable when SST are estimated; ii) only the warmer range of temperatures for the left-coiling populations would be represented in the samples; and iii) a remarkable warming inside the westernmost Mediterranean during the summer stage mixing cold and warm assemblages in bottom sediments. In addition, these results in combination with those derived from the isotopic analyses (δ¹⁸O) in G. bulloides tests, suggest that during the Pliocene-Pleistocene transition two different populations of left-coiling Neogloboquadrina could have existed with different environmental requirements: one, derived from late Miocene-Pliocene (i.e., left-coiling N. acostaensis group), and another being the ancestor of the modern N. pachyderma (left-coiling). Similarity analyses were achieved in order to locate the position of the core-tops with the assemblages most analogous to those of the fossil samples containing left-coiling Neogloboquadrina. The results reinforce the idea that the presence of these forms in the Mediterranean during the intervals studied would be related mainly to the input of cold waters from the North Atlantic during glacial stages, although it could be secondarily favored by the establishment of upwelling conditions, as in the present-day North Alboran waters.     

The oxygen isotope composition of planktonic foraminifera from the Guaymas Basin,Gulf of California: Seasonal,annual, and interspecies variability

Sediment trap samples collected over a seven-year period (February 1991–October 1997) from Guaymas Basin in the Gulf of California were used to study the oxygen isotope composition of five species of planktonic foraminifera, Globigerinoides ruber (white), Globigerina bulloides, Neogloboquadrina dutertrei, Pulleniatina obliquiloculata, and Globorotalia menardii. The δ¹⁸O data were analyzed for temporal and interspecies variability and were compared to local hydrography to evaluate the use of each species in reconstructing past oceanographic applications. The two surface dwelling species, G. ruber and G. bulloides displayed the lowest δ¹⁸O values (～ 0.0 to ? 5.0‰), while δ¹⁸O values for the thermocline dwelling N. dutertrei, P. obliquiloculata, and G. menardii were higher (～ 0.0 to ? 2.0‰). The δ¹⁸O of G. ruber most accurately records measured sea surface temperatures (SSTs) throughout the year. G. bulloides δ¹⁸O tracks SSTs during the winter–spring upwelling period but for the remainder of the year records slightly colder, subsurface temperatures. The difference between the δ¹⁸O of the surface dwelling species, G. ruber and G. bulloides, and that of the thermocline dwelling species, N. dutertrei, P. obliquiloculata, and G. menardii, was used to estimate the surface to thermocline temperature gradient. During the winter these δ¹⁸O differences are small (～ 0.50‰) reflecting a well-mixed water column. These interspecies δ¹⁸O differences increase during the summer (～ 1.90‰) in response to the strong thermal stratification that exists at this time of year.     

Estimating Pliocene sea-surface temperatures in the Mediterranean: An approach based on the modern analogs technique

The possibilities of adapting the Modern Analogs Technique (MAT) based on planktonic foraminifers for estimating sea-surface temperatures (SST) in the Mediterranean during the Pliocene are discussed in this article. The calibration database used comprises 684 core-top samples distributed in the North Atlantic and the Mediterranean. MAT estimates show an imperceptible bias (− 0.03 °C; σ = 0.59) and a low mean error of estimates (0.42 °C; σ = 0.42) when applied over the samples of the calibration dataset. The procedure used for assimilating the Pliocene taxonomic categories to those of the modern assemblages results in an increase from 17 to 40 in the number of samples showing an error > 2 °C when applied over the calibration database. However, the precision of MAT does not diminish when these samples are removed from the dataset. This methodology was used for obtaining SST estimates of late-middle Pliocene–earliest Pleistocene samples from ODP-site 975 (Menorca area), which have close modern analogs within the calibration database. In order to compare this technique with an additional proxy, we measured also δ¹⁸O values of G. bulloides tests from these samples. The results obtained show a good agreement on the whole, which corroborates the validity of the technical approach proposed.     

Wind and temperature controls on Alexandrium blooms (2000–2007) in Thau lagoon (Western Mediterranean)

Since 1998, blooms of Alexandrium catenella/tamarense in the lagoon of Thau developed regularly each autumn, reaching a maximum of several millions cells per liter in 2004. By contrast, spring blooms occurred only twice (in 2000 and 2007). During these periods, sea surface temperatures (SST) and the wind patterns appear to impact the bloom occurrences much more than the apparent limiting resources such as inorganic nutrients. The analysis of SST and wind from April to June and September to November (from 2000 to 2007) indicates first that there has to be an initial wind stress in order to resuspend the cysts buried in the sediment. Blooms then occur after a period of weak winds (<4 m s⁻¹) and of stable SST close to 20 °C (±2 °C). Those conditions appear to be most favorable for germination of Alexandrium cysts and its ensuing vegetative growth. This period of stability (a few days to a few weeks) allows the development of the inoculum from the cyst's germination, its cohesion because of reduced hydrodynamics, and development of vegetative cells that are sensitive to agitation. Strong winds during 1–2 day periods can interrupt the bloom dynamics by dispersing (advection due to southeasterly winds) and/or eliminating (turbulence due to northwesterly winds) the vegetative cells. In the spring, under the same conditions of optimal SST, strong wind episodes dominate and those, as well as biological factors very likely lead to a lower occurrence of blooms relative to the fall situation.     

Foraminiferal oxygen and carbon isotopes during the last 34 kyr off northern Japan,northwestern Pacific

Oxygen and carbon isotopes of foraminifera were analyzed in core PC4, water depth 1366 m, off northern Japan, near the east side of the Tsugaru Strait (130 m depth) between the open northwestern Pacific Ocean and the Japan Sea. At present, the site is at the confluence of the Tsugaru Warm Current which flows eastwards out of the Sea of Japan through the Tsugaru Strait, the subarctic Oyashio Current and the subtropic Kuroshio Current. During the Last Glacial Maximum (LGM), the Oyashio Current penetrated further to the South and outflow from the Japan Sea was restricted by glacio-eustatic sea level lowering.The isotopic values of the planktic foraminifer Neogloboquadrina pachyderma (sinistral) and the benthic foraminifer Uvigerina akitaensis reflect rapid millennial-scale paleoceanographic changes between 34 and 6 ka. Hydrographic changes during deglaciation were related to events at high northern latitudes, but Holocene hydrographic changes were dominated by local effects, such as the development of the outflow of the Tsugaru Warm Current. High values of planktic δ¹⁸O during the LGM reflect the southward advance of the Oyashio Current. These values decreased by 0.3‰ from 19.4 to 18.9 ka, then increased by 0.5‰ at 18 ka, with highest values between 17.5 and 15 ka. The δ¹⁸O oscillations between 19.4 and 15 ka may reflect millennial-scale warm–cold oscillations during Heinrich event 1. Planktic microfossil data indicate that cold Oyashio waters flowed from the northwestern Pacific into the Japan Sea via the Tsugaru Strait between 17 and 16 ka, consistent with the occurrence of the highest planktic δ¹⁸O values in core PC4. Planktic δ¹⁸O values rapidly decreased by 0.9‰ at 15 ka, possibly reflecting the effects of both a rapid increase in fresh water flux and rising temperatures in the subarctic North Pacific. During the Younger Dryas, cold event planktic δ¹⁸O values increased by 0.5‰, followed by a gradual decrease by 1‰ from the early to middle Holocene, reflecting a gradual increase in eastward outflow via the Tsugaru Strait with sea level rise. Both planktic and benthic foraminiferal δ¹³C values oscillated between 34 and 10 ka, at relatively large amplitudes (about 0.5‰), then remained relatively stable during the last 10 kyr. Several negative planktic and benthic (∼ − 0.7‰) δ¹³C excursions were present in sediment dated between the precipitation of secondary carbonates during episodic methane release possibly associated with methane release from continental margin sediments.     

Climate variability and Dinophysis acuta blooms in an upwelling system

Dinophysis acuta is a frequent seasonal lipophilic toxin producer in European Atlantic coastal waters associated with thermal stratification. In the Galician Rías, populations of D. acuta with their epicentre located off Aveiro (northern Portugal), typically co-occur with and follow those of Dinophysis acuminata during the upwelling transition (early autumn) as a result of longshore transport. During hotter than average summers, D. acuta blooms also occur in August in the Rías, when they replace D. acuminata. Here we examined a 30-year (1985–2014) time series of D. acuta from samples collected by the same method in the Galician Rías. Our main objective was to identify patterns of distribution and their relation with climate variability, and to explain the exceptional summer blooms of D. acuta in 1989–1990. A dome-shaped relationship was found between summer upwelling intensity and D. acuta blooms; cell maxima were associated with conditions where the balance between upwelling intensity and heating, leading to deepened thermoclines, combined with tidal phase (3 days after neap tides) created windows of opportunity for this species. The application of a generalized additive model based on biological (D. acuta inoculum) and environmental predictors (Cumulative June–August upwelling CUI_JJA, average June–August SST_JJA and tidal range) explained more than 70% of the deviance for the exceptional summer blooms of D. acuta, through a combination of moderate (35,000–50,000 m³ s⁻¹ km⁻¹) summer upwelling (CUI_JJA), thermal stratification (SST_JJA > 17 °C) and moderate tidal range (∼2.5 m), provided D. acuta cells (inoculum) were present in July. There was no evidence of increasing trends in D. acuta bloom frequency/intensity nor a clear relationship with NAO or other long-term climatic cycles. Instead, the exceptional summer blooms of 1989–1990 appeared linked to extreme hydroclimatic anomalies (high positive anomalies in SST and NAO index), which affected most of the European Atlantic coast.     

Changes in style and intensity of production in the Southeastern Atlantic over the last 70,000 yr

Accomplishing reliable paleo-reconstructions of productivity and upwelling conditions in eastern boundary current systems requires the use of cores collected in a basin-wide spatial pattern. Based on diatom assemblage analysis and the concentration and the bulk biogenic components of three gravity cores recovered from the Benguela Upwelling System (BUS) between 19° and 25°S, I describe rapid paleoceanographic changes that occurred during the last 70 ka B.P. in the southeastern Atlantic. The pattern of biogenic production and accumulation differs to varying degrees among the three core sites along the SW African coast. The highest sedimentation and accumulation rates at 25°S off Lüderitz conform with the present-day, well-known pattern of highest productivity and most intense coastal upwelling. Highest diatom values at 25°S during MIS3 points to more intense upwelling due to the combination of strong seaward-extending upwelling filaments, shoaling of the upwelled water, and the influence of silicate-rich waters of Antarctic origin. Productivity decreased along the central BUS throughout MIS2, when the siliceous–calcareous productivity regime shifted toward a system dominated by calcareous producers. Although intensity and strength of winds created adequate conditions for upwelling during MIS2, diatom production decreased. The complete replacement of the upwelling-associated diatom flora by a non-upwelling-related diatom community during MIS1 reflects weakened upwelling, weakened seaward extension of the upwelling filaments, and dominance of warmer surface waters. Combining changes in the composition of the diatom assemblage and variations of the bulk biogenic components allows for reliable reconstruction of paleoproductivity and upwelling changes for the SE Atlantic during the last 70 ka B.P.     

A new stage 3 millennial climatic variability record from a SW France speleothem

We present a new high resolution speleothem stable isotope record from the Villars Cave (SW-France) that covers part of marine isotope stage (MIS) 3. The Vil14 stalagmite grew between ~ 52 and 29 ka. The δ¹³C profile is used as a palaeoclimate proxy and clearly shows the interstadial substages 13, 12 and 11. The new results complement and corroborate previously published stalagmite records Vil9 and Vil27 from the same site. The Vil14 stalagmite chronology is based on 12 Th-U dating by MC-ICP-MS and 3 by TIMS. A correction for detrital contamination was done using the ²³⁰Th/²³²Th activity ratio measured on clay collected in Villars Cave. The Vil14 results reveal that the onset of Dansgaard–Oeschger (DO) events 13 and 12 occurred at ~ 49.8 ka and ~ 47.8 ka, respectively. Within uncertainties, this is coherent with the latest NorthGRIP time scale (GICC05-60 ka) and with speleothem records from Central Alps. Our data show an abrupt δ¹³C increase at the end of DO events 14 to 12 which coincides with a petrographical discontinuity probably due to a rapid cooling. As observed for Vil9 and Vil27, Vil14 growth significantly slowed down after ~ 42 ka and finally stopped ~ 29 ka ago where the δ¹³C increase suggests a strong climate deterioration that coincides with both North Atlantic sea level and sea surface temperature drop.     

Holocene and Late Pleistocene climate in the sub-Mediterranean continental environment: A speleothem record from Poleva Cave (Southern Carpathians,Romania)

The PP10 stalagmite from Poleva Cave provides a Late Pleistocene and Holocene isotopic record characteristic for the SW of Romania, a sub-Mediterranean climatic region. The speleothem was dated by eight TIMS and one alpha U-series dates which showed that it was precipitated between ∼ 75 ka and ∼ 2 ka with at least two hiatuses. The basal sector of the stalagmite showed a slow-growing regime of ∼ 0.26 cm/ka, while the upper one grew relatively fast with about 5 cm/ka. The temporal resolution for the isotopic sampling is thus ∼ 2 ka/sample for the lower sector, and ∼ 150 years/sample for the upper one. The relationship between δ¹⁸O and temperature was found positive. The isotopic record of the lower sector shows two marked cold intervals during ∼ 67 and 58 ka and ∼ 40–35 ka, respectively, which correlate well with the Villars and Soreq records. The upper sector record is so far the most detailed Holocene isotopic record in Romania and the only one available for the regions located at the exterior of the Carpathians Range. The signal shows a gradual warming after the GS1 event punctuated by several cold events at ∼ 8, 7.2 and 4.2 ka and also by warm oscillations centered at about 5.2 and 3.3 ka. The results seem to indicate that if the North-Atlantic first-order signals may extend well to the south-eastern Europe, their amplitude and general trend may be diminished by the interferences with the Mediterranean circulation.     

Carbon and oxygen isotope evidence for high-frequency (104–105 yr) and My-scale glacio-eustasy in Middle Pennsylvanian cyclic carbonates (Gray Mesa Formation), central New Mexico

We combine cyclo- and sequence stratigraphy along with whole rock δ¹³C and conodont apatite δ¹⁸O analysis to document high-frequency (10⁴–10⁵ yr) and My-scale sea-level changes for the Middle Pennsylvanian (Desmoinesian or Moscovian) Gray Mesa Formation of central New Mexico. Approximately 75 subtidal cycles (1–8 m) are grouped into 4 1/2 My-scale depositional sequences (40–80 m). About 50% of the cycles show evidence of prolonged subaerial exposure at cycle tops with the development of calcretes, diagenetic mottling, and regolith intraclasts. High-resolution δ¹³C analysis of whole rock limestones across nine of the cycles indicates that the cycle tops were diagenetically altered by isotopically light, meteoric fluids during sea-level fall and lowstand. These δ¹³C trends support the interpretation that high-frequency sea-level changes were responsible for cycle development.Conodont apatite δ¹⁸O values from sampled cycles indicate that the high-frequency sea-level changes were driven by glacio-eustasy combined with changes in surface seawater temperature (SST). δ¹⁸O values from conodont apatite, spanning parts of three depositional sequences indicate that My-scale glacio-eustasy and/or SST changes controlled sequence development. δ¹⁸O shifts indicate that the magnitudes of 10⁴–10⁵ yr glacio-eustasy were between ～ 55 and 170⁺ m combined with tropical SST changes of ～ 1.5°–6 °C. Calculated My-scale glacio-eustatic oscillations were between ～ 60 and 140 m with SST changes of < 3.5 °C. The most plausible driver for the My-scale paleoclimate changes is long-period obliquity (～ 1.2 My) variations. These calculated high-frequency, glacio-eustatic values are similar or greater than Pleistocene values, and lie within the range estimated for other Middle Pennsylvanian successions using a variety of independent eustatic proxies. The similarity in range of magnitudes between high-frequency and My-scale sea-level changes combined with the large differences in magnitudes between individual high-frequency sea-level oscillations helps explain the lack of systematic cycle-stacking patterns within these Pennsylvanian icehouse sequences.     

New insights into the pattern,timing, and duration of the evolutionary origin of the foraminiferal genus Hantkenina

A high-resolution study of the Tethyan Contessa Highway reference section (Gubbio, Italy) has produced a detailed, well-constrained and more complete pattern of the gradual evolutionary origin of the foraminiferal genus Hantkenina from Clavigerinella than hitherto shown. A nine-stage pattern is presented for this evolutionary lineage. The related time frame and duration are assessed using the astronomical calibration of the Contessa Highway record. The evolution of Clavigerinella caucasica to Hantkenina gohrbandti, the real ancestor of Hantkenina, leads in 333 kyr. The H. gohrbandti first appearance is dated at 45.564 Ma. Our findings show that during the middle Eocene, from 46 to 45.5 Ma and with a major pulse centered at 45.521 Ma deep-mesopelagic planktonic foraminifera may have developed different morphological modifications to tolerate temporary exceptionally stressful environmental conditions. These conditions are principally related to poorly oxygenated and usually food-poor waters. This suggests that sustained and expanded oxygen minimum zone conditions and associated enhanced eutrophication might have occurred during that time.     

Inorganic and organic nitrogen uptake by the toxigenic diatom Pseudo-nitzschia australis (Bacillariophyceae)

The nitrogen uptake capabilities of the toxigenic diatom Pseudo-nitzschia australis (Frenguelli), freshly isolated from Monterey Bay California, were examined in unialgal laboratory cultures at saturating photosynthetic photon flux densities (100 μmol photons m⁻² s⁻¹) and 15 °C. The kinetics of nitrogen (nitrate, ammonium, urea and glutamine) uptake as a function of substrate concentration were estimated from short (20.5 min) incubations using the ¹⁵N-tracer technique. Based on the estimated maximum specific uptake rates and measures of N affinity (the initial slope of the uptake versus nutrient concentration curve), nitrate is the preferred nitrogen substrate, followed by glutamine and ammonium, which are equivalent. Rates of urea uptake by P. australis did not saturate at concentrations as high as 36 μg-at N L⁻¹, and urea uptake as a function of concentration could not be described by Michaelis–Menten kinetics over the concentration gradient tested. Although there is a clear preference for nitrate at equivalent concentrations (compared to ammonium, urea, and glutamine), these laboratory results demonstrate the capability of this pennate diatom to utilize both inorganic and organic forms of nitrogen, supporting field observations that P. australis blooms during both upwelling and non-upwelling conditions off the west coast of North America. Substantial differences in the nitrogenous nutrition of P. australis can be expected in these environments, and anthropogenic inputs of N substrates such as ammonium and urea can support its growth, and may contribute significantly to both harmful diatom blooms and the maintenance of seed populations at non-bloom abundances, particularly during periods of reduced or absent upwelling.