Variability in the vertical structure of the water column and paleoproductivity reconstruction in the central-western Mediterranean during the Late Pleistocene

A sedimentary sequence spanning Marine Isotope Stage (MIS) 6 to MIS 2 in core LC07, recovered in the central Mediterranean, has been investigated in order to produce a high-resolution paleoceanographic reconstruction. The changes in productivity deduced from calcareous plankton relative abundances and independently confirmed by the Ba_XS fluctuations are linked to the stability of the water column which is mainly controlled by the water mass temperature. During glacial intervals, productivity was generally enhanced. Oligotrophic and warmer water masses with a deepened seasonal thermocline can be inferred for most of MIS5. The magnetic properties of the sediment show increased occurrences of North Africa dust in the central Mediterranean during cold phases, likely as a consequence of a more efficient erosive process triggered by southward displacement of the intertropical convergence zone. Although increases in both productivity and Saharan dust occurred during cold periods, the atmospheric inputs do not seem to contribute significantly to the fertilization of primary producers. A Shannon Index curve has been used to tentatively synthesize the variations of calcareous nannofossil assemblages through the last 150 kyr. The assemblage diversity sharply increased coincident with the transition from the penultimate glacial to the last interglacial, subsequently low diversity was gradually reached again in the last glacial.     

The variation of upper ocean structure and paleoproductivity in the Kuroshio source region during the last 200 kyr

A core from the source region of the Kuroshio warm current (east of the Luzon Island) was analyzed using several proxies in order to study the variability of the Western Pacific Warm Pool (WPWP) during the last two glacial–interglacial cycles. Primary productivity (PP) variations were deduced from variations in the coccolith flora. Primary productivity was higher during glacial periods (the end of Marine Isotope Stage [MIS] 3, some periods in MIS 2 and 6), and decreased during interglacial periods (MIS 7, MIS 5e and probably MIS 5c–5d), with the lowest PP in MIS 5e. Variations in the δ¹³C difference in benthic and bulk carbonate, thus in the vertical gradient of δ¹³C in dissolved inorganic carbon (Δδ¹³C_{C. wuellerstorfi–N. dutertrei} and Δδ¹³C_{C. wuellerstorfi–coccolith}) coincided with the PP changes, showing that export productivity was low during interglacial periods (MIS 7, MIS 5e and Holocene) and high during glacial periods (MIS 6, probably MIS 5c–5d, late MIS 4 and late MIS 3). Comparison of foraminiferal carbonate dissolution indicators and PP changes reveals that nannofossil assemblage in core Ph05-5 is not sensitive to carbonate dissolution intensity. The depth of the thermocline (DOT) was estimated from planktonic forminiferal assemblages, and was relatively greater during interglacial periods (MIS 7, MIS 5e, probably MIS 5c and Holocene) than during glacials (middle MIS 6, probably MIS 5b and 5d, some periods in MIS 4, MIS 3 and MIS 2). Good coherence between the paleoproductivity records and the DOT suggests that the DOT changes could be the primary control factor in changes of paleoproductivity, and the glacial high productivity in the Kuroshio source region could be associated with a global increase of nutrient concentration in the intermediate waters that upwelled into the photic zone. The low CO₂ values derived for intervals of high productivity and a relatively shallow DOT suggest that the changes in biological productivity and DOT in the equatorial Pacific could have modified atmospheric CO₂ concentrations. High Sea Surface Temperatures (SSTs) during the warm MIS 5e in combination with intensified monsoonal rain fall could have resulted in a more intense stratification of the upper waters, resulting in low nutrient supply to the surface waters and a resulting decrease in productivity.     

Radiolarian and sedimentologic paleoproductivity proxies in late Pleistocene sediments of the Benguela Upwelling System,ODP Site 1084

The changing composition of radiolarian faunas from late Neogene deep-sea sediments has been used in recent years as a proxy for changes in marine paleoproductivity. We examine radiolarian faunas, organic carbon content (TOC), opal and coarse-fraction components over the last 270,000 years in sediments from ODP Hole 1084A, drilled in a high productivity upwelling region within the Benguela Upwelling System off the west coast of Africa. Age control is provided by stable oxygen isotope measurements of benthic foraminifera. Prior research has established that late Pleistocene glacial intervals in this upwelling system generally had higher productivity than interglacials. The radiolarian WADE (water-depth ecology) paleoproductivity index correlates well with TOC and opal in these samples, and all three parameters change in synchrony with the benthic isotope curve over all but the MIS 5e–6 time interval. WADE inferred productivity is significantly higher in glacials than interglacials. We conclude that the WADE index is a useful proxy for paleoproductivity at this location, as are also opal and organic carbon accumulation rates. Carbonate and carbonate based indices such as the accumulation rate of benthic foraminifera (BFAR) by contrast do not correlate well either to productivity indices or to the glacial–interglacial cycle, and are interpreted to primarily reflect carbonate dissolution.     

Oxygen minimum expansion in the Sulu Sea,western equatorial Pacific,during the last glacial low stand of sea level

The Sulu Sea in the western equatorial Pacific is presently a shallowly-silled, dysaerobic, deep-marine basin. Deep waters in the Sulu Sea are ventilated through a single sill at 420 m depth which connects it to the China Sea. Benthic and planktonic foraminiferal oxygen and carbon isotope records, benthic and planktonic foraminiferal census data and total organic carbon measurements have been used to evaluate changes in water mass conditions in the Sulu Sea between the last glacial maximum (18,000 yrs. B.P.) and the present day.An increase in the abundance of the planktonic foraminiferaNeogloboquadrina dutertrei and relatively light planktonic foraminiferal δ¹⁸O values suggest that during the last glacial maximum surface water salinities were reduced in the Sulu Sea. Enhanced isolation of the basin due to glacio-eustatic lowering of sea level and reduced surface salinities resulted in stagnation of deep water and an expansion of the mid-water oxygen minimum layer. Increased organic carbon preservation at mid-water depths occurs at this time. Benthic carbon isotope data and an increase in the abundance of benthic foraminiferal species considered to prefer low oxygen environments support the conclusion of an oxygen-minimum expansion at mid-water depths during the last glacial maximum. At water depths greater than 4000 m, bottom waters appear to have maintained some degree of oxygenation during the last glacial maximum. Stronger Pacific Ocean trade winds at this time may have caused the influx of denser Celebes Sea surface water into the southern part of the Sulu Sea. The slow sinking of this water would have then ventilated bottom waters in this part of the basin.At the transition from glacial to interglacial conditions, rising sea level caused denser water to flow over the deepest sill into the Sulu Sea. Vertical circulation increased, resulting in a greater downward flux of oxygen and a dissipation of the oxygen minimum. Continued post-glacial sea level rise caused periodic ventilation of deep water until the present dysaerobic conditions were established.     

Late Quaternary history of atmospheric and oceanic circulation in the eastern equatorial Pacific

Four radiolarian assemblages have been defined in recent seafloor sediments of the equatorial Pacific Ocean. The distribution of these assemblages corresponds to the modern pattern of oceanic circulation and water mass structure in this region: the eastern Pacific shallow permanent thermocline and the Equatorial Undercurrent; Peru Current upwelling and the oxygen minimum; the subtropical water mass; warm western tropical water and the North Equatorial Countercurrent. In twelve cores chosen to transect the region both longitudinally and latitudinally, the distribution of these four assemblages has been reconstructed for six time-intervals during the last 127,000 years: 18,000 B.P. (glacial Stage 2); 36,000 B.P. and 52,000 B.P. (interstadial Stage 3); 65,000 B.P. (glacial Stage 4); 82,000 B.P. and 120,000 B.P. (interglacial Stage 5). Atmospheric and oceanic circulation changes through time have been inferred from the reconstructed microfossil assemblage distributions. Changes in assemblage distributions indicate that variations in intensity, direction and mean position of the tradewinds caused marked changes in the oceanic circulation patterns through the last glacial cycle.Near the end of interglacial Stage 5, the disappearance of the North Equatorial Countercurrent from the eastern Pacific suggests that the mean position of the tradewinds was shifted to the south approximately 5° of latitude relative to the modern position, so that the Northeast trades prevented the flow of the North Equatorial Countercurrent into the eastern Pacific. Near the end of interstadial Stage 3, a change in wind direction occurred from predominantly zonal winds, which enhance equatorial divergence and surfacing of the Equatorial Undercurrent, to more meridional winds, which enhance coastal upwelling associated with the Peru Current.In the tropical Pacific Ocean, late Quaternary changes in atmospheric and oceanic circulation are linked with times of continental ice sheet growth in the Northern Hemisphere (i.e., the interglacial-to-glacial transitions across oxygen isotope stage boundaries $\text{5}\text{4}$ and $\text{3}\text{2}$). The major changes in circulation seem to occur a few thousand years in advance of the glacial episodes, at or near periods of ice sheet growth. This relationship indicates that changes in atmospheric circulation in the tropics led and influenced the development of conditions suitable for polar and continental ice sheet growth in the Northern Hemisphere.     

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.     

Distribution of ostracod and benthic foraminiferal assemblages during the last 550 kyr in the East-Corsica basin,western Mediterranean Sea: A paleo-environmental reconstruction

The study of Quaternary sediments covering the last 550 kyr from the borehole GDEC-4-2, East-Corsica basin, was performed through the study of benthic meiofaunal assemblages (ostracoda and benthic foraminifera) whose distribution responds to climate changes, and glacial–interglacial variability in particular. The interglacial ostracod group is mainly composed of the Argilloecia acuminata, Cytheropteron alatum, Henryhowella sp., Polycope sp. and Cytherella sp., which indicate an enhanced surface productivity and possibly high bottom water temperature during these warm intervals. Benthic foraminifera, such as Hoeglundina elegans, Sigmoilopsis schlumbergeri, Bigenerina nodosaria and Hyalinea balthica are predominant during interglacial periods and also show an increasing surface productivity associated to warm intervals. Increase in surface productivity would contribute to an enhanced export of organic matter to sea floor at the GDEC-4-2 site during interglacial periods. The bottom temperature drop coupled with the important sedimentary inputs (associated to a significant quantity of refractory organic matter) during glacial periods was highlighted by the predominance of ostracoda, such as Paradoxostoma sp., Cytheropteron testudo, Bathycythere vanstraateni, Macrocypris sp., Echinocythereis echinata and benthic foraminifera as Bolivina spathulata, Bulimina costata, and Bulimina marginata. The distribution of some benthic foraminifera species describing a turnover system allowed the characterization of the last three climate cycles (MIS 8–9, MIS 6–7, MIS 1–5) probably marked by changes of bottom water paleoceanographic conditions in terms of temperature, salinity, bottom current intensity and surface productivity. The cyclic variations of the allochthonous group recorded over the last 550 kyr suggest a major role played by the eustatic factor, with the latter possibly favouring down slope transport. The distribution of this group allowed thus establishing a link between shelf-basin sediment transfer and climate, with enhanced shelf-basin sediment transfer during glacial low stands.     

Reconstruction of Indian monsoon variability over the past 230,000 years: Planktic foraminiferal evidence from the NW Arabian Sea open-ocean upwelling area

We determined the faunal composition and total number of tests (#/g) of planktic foraminifera (> 125 μm) in core KH00-05 GOA 6 near Oman in order to decipher monsoon-induced variability of oceanographic productivity in the open-ocean upwelling area in the northwest Arabian Sea. The core contains a continuous record of sedimentation over the last 230 kyr, with the age model based on oxygen isotope and accelerator mass spectrometry ¹⁴C dates. We focused on species (Globigerina bulloides and Globigerinita glutinata) typical for SW monsoonal upwelling and species typical for NE monsoon conditions (Neogloboquadrina incompta, Neogloboquadrina dutertrei, Globigerinoides ruber, and Globigerinoides sacculifer). The changes in relative abundance of these monsoonal indicators suggest that the open-ocean upwelling area was dominated by the SW monsoon during interglacial periods, but by the NE monsoon during glacial periods.Increases in total test abundance during glacial periods confirmed that the NE monsoon rather than SW monsoon contributes largely to planktic foraminiferal productivity in this area. We argue that three types of circumstances resulted in high productivity, with nine high productivity events occurring at a 23-kyr frequency. The first type caused high productivity events at 102 and 199 ka (interglacial periods), characterized by the dominance of upwelling species, indicating high productivity during strong SW monsoons, correlated with high July insolation at 45° N. An exceptional high productivity event occurred at 37 ka during interglacial marine isotope stage (MIS) 3, with contributions from both SW and NE monsoons. The second type of high productivity event occurred at 61, 147, and 175 ka, during glacial periods, characterized by dominance of NE monsoon species, and correlated with low January insolation at 45° N. In addition, a high productivity event at 85 ka (interglacial period) also was induced by enhanced NE monsoons. The last two high productivity events occurred during transitional periods from glacial to interglacial (MIS 6/5.5 and 2/1), were characterized by the replacement of NE monsoon species with upwelling species, and corresponded to abrupt climate warming, suggesting that they are related to both accelerated SW monsoon systems and reduced NE monsoon systems.     

Varieties of a single cosmopolitan diatom species associated with surface water masses in the North Pacific

A time series of 65 sediment trap samples, representing 48-month changes in diatom flora, and 62 sediment core-tops from the western North Pacific Ocean were analyzed. Varieties of a single diatom species were found to be characteristic of two surface water masses bounded by the Kuroshio Current. Thalassionema nitzschioides var. incurvata, var. inflata and var. parva dominate in warm saline waters in the Subtropical Gyre, while T. nitzschioides sensu stricto is abundant in less-saline waters northwest of this current. Time series data on percent abundances for the three T. nitzschioides varieties in the T. nitzschioides complex are compared with path variations of the current south of Japan. Inter-annual fluctuations in percent abundance are linked to changes in surface water masses over the sediment traps in association with migration of the Kuroshio paths. This finding makes the percent abundance a useful tool for deciphering path migrations of the Kuroshio Current in the geologic past. Down-core analyses of two sediment cores collected beneath the modern Kuroshio Current were performed. A sharp increase in percent abundances from 12 to 9 kyr BP, with an abrupt change around 11 kyr BP, records the previously reported transition of the Kuroshio Current from a predominantly offshore path to its modern path. Path migrations of the Kuroshio Current may be inferred from percent abundances of these diatoms, which comprise a new proxy for delineating the paleoclimate of the northwestern Pacific in relation to changes in northward heat transport from the western equatorial Pacific Ocean by the western boundary current.     

The biostratigraphy and paleobiology of Oligocene planktonic foraminifera from the equatorial Pacific Ocean (ODP Site 1218)

Planktonic foraminifera from a continuous Oligocene succession with clear magnetochronology and sediment cycles at Ocean Drilling Program Site 1218 (equatorial Pacific Ocean) were studied in the interval from 27 to 30 Ma. Paragloborotalia taxa are common and we examined their size, relative abundance, and stable isotopes. Multispecies stable isotope data indicate the depth habitats of Oligocene planktonic foraminifera and suggest that “Globoquadrina” venezuelana and Dentoglobigerina globularis were probably mixed-layer dwellers, with paragloborotaliids recording heavier δ¹⁸O signatures consistent with a thermocline habitat. Cyclic variations in the abundance of Paragloborotalia match eccentricity (100 kyr) variations in percent carbonate and δ¹³C, suggesting orbitally forced upwelling in the equatorial Pacific Ocean and that Paragloborotalia were responding directly to changes in surface water productivity. The high-resolution biostratigraphy calibrated to the magnetochronology constrains the extinction of Paragloborotalia opima which marks the top of Planktonic Foraminifera Biozone O5 (P21b) at 27.456 Ma. The highest occurrence of P. opima is associated with a 50% size decrease in Paragloborotalia pseudocontinuosa taxa within Chron 9n. In addition, we find the extinction of Chiloguembelina cubensis is consistent with other deep-sea sections within Chron 10n at 28.426 Ma marking the O4/O5 (P21a/P21b) boundary.     

Early Pleistocene glacial-interglacial radiolarian assemblages from the eastern equatorial Pacific

Polycystine radiolaria from ODP Hole 677A in the eastern equatorialPacific were examined at isotopically identified Early Pleistoceneglacial maxima and minima. Two distinct radiolarian assemblagesare recognized, characterizing glacial and interglacial optima.The Glacial Assemblage is characterized by high abundances ofTheocalyptra davisiana, Botryostrobus auritus, Anthocyrtidiumzanguebaricuim andHexacontium enthacanthum. The InterglacialAssemblage is characterized by Tetrapyle octacantha, Octapylestenozoa and Theocorythium vetulum. A comparison of these fossilassemblages with modern radiolarian distribution suggests thatthe Glacial Assemblage represents intensified upwelling of coldadvected water via the Eastern Pacific Boundary Current, whilstthe Interglacial Assemblage indicates climatic ameliorationin the eastern equatorial Pacific, with the prevalence of warm(>21C) tropical/subtropical surface waters. The recognitionof these radiolarian assemblages could be successfully appliedto studies of adjacent east Pacific areas where other palaeoecologicalindicators are lacking.     

Paleoceanographic conditions in the western Caribbean Sea for the last 560 kyr as inferred from planktonic foraminifera

Faunal analyses of planktonic foraminifera and upper-water temperature reconstructions with the modern analog technique are studied and compared to the magnetic susceptibility and gamma ray logs of ODP Core 999A (western Caribbean) for the past 560 kyr in order to explore changes in paleoceanographic conditions in the western Caribbean Sea. Long-term trends in the percentage abundance of planktonic foraminifera in ODP Core 999A suggest two hydrographic scenarios: before and after 480 ka. High percentage abundances of Neogloboquadrina pachyderma and Globorotalia inflata, low abundances of Globorotalia menardii and Globorotalia truncatulinoides, low diversity, and sea-surface temperatures (SST) under 24 °C are typical characteristics occurring from 480 to 560 ka. These characteristics suggest a “shallow” well-oxygenated upper thermocline and the influx of nutrients by either seasonal upwelling plumes and/or eddy-mediated entrainment. The second scenario occurred after 480 ka, and it is characterized by high and fluctuating percentage abundances of Neogloboquadrina dutertrei, G. truncatulinoides, G. menardii, Globigerinita glutinata, Globigerinella siphonifera, and Globigerinoides ruber; a declining trend in diversity; and large SSTs. These characteristics suggest a steady change from conditions characterized by a “shallow” thermocline and chlorophyll maximum to conditions characterized by a “deep” thermocline (mainly during glacial stages) and by more oligotrophic conditions. The influence of the subtropical North Atlantic on the upper thermocline was apparently larger during glacial stages, thus favoring a deepening of the thermocline, an increase in sea-surface salinity, and a dramatic reduction of nutrients in the Guajira upwelling system. During interglacial stages, the influx of nutrients from the Magdalena River is stronger, thus resulting in a deep chlorophyll maximum and a fresher upper ocean. The eddy entrainment of nutrients is the probable mechanism responsible of transport from the Guajira upwelling and Magdalena River plumes into ODP 999A site.     

Late Quaternary paleoproductivity and deep water circulation in the eastern South Atlantic Ocean: Evidence from benthic foraminifera

Two late Quaternary sediment cores from the northern Cape Basin in the eastern South Atlantic Ocean were analyzed for their benthic foraminiferal content and benthic stable carbon isotope composition. The locations of the cores were selected such that both of them presently are bathed by North Atlantic Deep Water (NADW) and past changes in deep water circulation should be recorded simultaneously at both locations. However, the areas are different in terms of primary production. One core was recovered from the nutrient-depleted Walvis Ridge area, whereas the other one is from the continental slope just below the coastal upwelling mixing area where present day organic matter fluxes are shown to be moderately high. Recent data served as the basis for the interpretation of the late Quaternary faunal fluctuations and the paleoceanographic reconstruction.

During the last 450,000 years, NADW flux into the eastern South Atlantic Ocean has been restricted to interglacial periods, with the strongest dominance of a NADW-driven deep water circulation during interglacial stages 1, 9 and 11. At the continental margin, high productivity faunas and very low epibenthic δ¹³C values indicate enhanced fluxes of organic matter during glacial periods. This can be attributed to a glacial increase and lateral extension of coastal upwelling. The long term glacial-interglacial paleoproductivity cycles are superimposed by high-frequency variations with a period of about 23,000 yr. Enhanced productivity in surface waters above the Walvis Ridge, far from the coast, is indicated during glacial stages 8, 10 and 12. During these periods, cold, nutrient-rich filaments from the mixing area were probably driven as far as to the southeastern flank of the Walvis Ridge.     

Late Neogene nannofossil biostratigraphy and paleoceanography of the northeastern Gulf of Mexico and adjacent areas

Two 305 m cored sections from the northwest Florida continental shelf contain a nearly complete record of late Neogene hemipelagic sedimentation. One of the sites, south and east of De Soto Canyon, is isolated from terrigenous sediment except for sediment transported in suspension. This site contains a continuous record from the late Miocene to the Recent. The second site, on the western rim of De Soto Canyon, is more expanded and continuous from the late Pliocene to the Recent. A hiatus separates the late Pliocene from the middle Miocene. Six prominent nannofossil biohorizons were recognized within the Pleistocene, seven within the Pliocene, and three within the Miocene; in addition one biohorizon marks the base of the Pleistocene and another the base of the Pliocene.Nearly all carbonate in the sediment is pelagic. Terrigenous detrital sedimentation was controlled by glacioeustatic sea level fluctuations during the Pleistocene, and sea level changes are probably responsible for fluctuations in the ratio of pelagic carbonate to clayey detritus in pre-Pleistocene sediments also. Carbonate content, coarse fraction percent, and relative abundances of environmentally sensitive nannoplankton species suggest important paleoceanographic changes in the northeastern Gulf of Mexico and adjacent areas. Fluctuations in the relative abundance of the solution-resistant coccoliths of the genusCyclococcolithus indicate that waters at a depth of 600–1000 m were more corrosive during the late Miocene than they are today. The decrease in carbonate dissolution during the late Miocene probably was a response to gradual constriction of the Central American passage and the consequent restriction of flow of corrosive water from the Pacific. Short term fluctuations in dissolution during the Pliocene and Pleistocene are related to climatic cycles.Productivity variations in the surface waters, recorded mainly by the relative abundance of small and large morphotypes of closely related coccolith species, indicate that productivity increased during the Pliocene, but the most dramatic change — a major oceanwide increase in productivity — occurred during the Pleistocene, during and just prior to the Jaramillo magnetic event about 0.9 m.y. ago. Surprisingly the late Miocene Messinian event did not leave a significant imprint in the northeastern Gulf of Mexico.     

High frequency palaeoceanographic changes during the past 140?000 yr recorded by the organic matter in sediments of the Iberian Margin

Biogenic records of the marine palaeoproductivity (carbonates, organic carbon, and C₃₇ alkenones) and the molecular stratigraphy of past sea surface temperatures (SSTs; U₃₇^K′) were studied at high resolution in two cores of the Iberian Margin. The comparison of these records indicates that the oceanographic conditions switched abruptly during the past 160 kyr between three kinds of regimes. A first regime with high (17-22°C) SST and low productivity typifies the interglacial periods, marine isotopic stages (MIS) 5 and 1. Several periods during MIS 6, 2, and the terminations II and I are characterised by about 4-5°C colder SST and a higher organic matter accumulation, both of which define the second regime. This anticorrelation between SST and marine productivity suggests that these variations are related to the intensity of the coastal upwelling. By contrast with this upwelling behaviour, extremely low biological productivity and very cold SST (6-12°C) occurred during short phases of glacial MIS 6, 4, and 2, and as abrupt events (≈1 kyr or less) during MIS 3. The three oceanographic regimes are consistent with micropalaeontological changes in the same cores based on foraminifera and diatoms.The general trend of these hydrologic changes follows the long-term glacial/interglacial cycle, but the millennium scale variability is clearly related to Heinrich events and Dansgaard-Oeschger cycles. Strengthening of the upwelling corresponds probably to an intensification of the subtropical atmospheric circulation over the North Atlantic which was influenced by the presence of continental ice sheets. However, extreme glacial conditions due to massive discharges of icebergs interrupted the upwelling. Interestingly, both terminations II and I coincided with strong but transient intensification of the upwelling.     

Paleoenvironmental changes during sapropel 19 (i-cycle 90) deposition: Evidences from geochemical,mineralogical and micropaleontological proxies in the mid-Pleistocene Montalbano Jonico land section (southern Italy)

An integrated micropaleontological, geochemical and mineralogical study has been performed across the mid-Pleistocene sapropel 19 (i-cycle 90) from the Montalbano Jonico land section (southern Italy), to reconstruct the paleoenvironmental conditions at time of its formation. The sapropel interval is characterized by two oxygen depletion phases (phase A and C) interrupted by a temporary re-oxygenation interval (phase B). The beginning and the end of sapropel deposition are dated at 957 ± 0.81 kyr and 950 ±0.86 kyr respectively. The duration of the interruption is estimated to 0.350 ± 0.32 kyr. The multiproxy approach highlights that deposition of sapropel 19 reflects a period of enhanced freshwater runoff induced by a wetter climate. As a consequence of a more efficient fluvial erosion, a higher terrigenous input, mostly ascribable to a southern Apennines source, and an increased turbidity of surface waters accompanied most of sapropel deposition. Biotic and abiotic proxies document that different paleoenvironmental conditions occur through phases A–C. The beginning of phase A is characterized by warm on-land paleoclimate as well as warm and oligotrophic surface water conditions. During the upper part of phase A temperature starts decreasing and surface waters appear more productive. This change probably represents the prelude to cooler and drier conditions characterizing phase B, which displays a river supply reduction and an eolian input increase (Sahara dust). During phase C the restored depleted oxygen environment at the bottom sediments is clearly coupled with the re-establishment of humid conditions and increased river supply. At the same time, enhanced mixing of water column, a cooler paleoclimate, and increased productivity of surface waters are recorded, the latter likely favored by the enhanced mixing of water column and also increased delivery of land-derived nutrients. The end of phase C is marked by a restored “normal” run-off. Enhanced productivity in surface waters and low oxygen conditions at the bottom sediments persist slightly above phase C. The overall results suggest that the onset of sapropel deposition is related to water stratification that caused low oxygen exchanges with the sea-bottom. Although enhanced productivity characterizes most of the sapropel deposition it was not the primary factor triggering sapropel deposition.     

Glacial–interglacial deep-water changes in the NW Pacific inferred from single foraminiferal δO and δC

Oxygen and carbon isotope values of single benthic foraminiferal tests in a core from the Shatsky Rise, NW Pacific Ocean, show greater intra-horizon variance during the Holocene than during the Last Glacial Maximum (LGM). This greater variance is caused by the introduction of glacial specimens some 20 cm upward from their original deposition layer due to bioturbation. In contrast, foraminiferal populations belonging to glacial layers do not include Holocene specimens. The difference in direction of bioturbation greatly modifies climate information in horizons formed during and after deglacial events. After omitting glacial specimens from Holocene sediments, the glacial–interglacial difference in δ¹⁸O suggests that Pacific deep-water temperature changed by 2.4–3.8°C at the most. The δ¹³C values suggest that nutrient concentration was higher during the LGM than the Holocene. The glacial deep North Pacific Ocean apparently was influenced by cold deep waters of southern origin.     

Coccolithophores in the equatorial Atlantic Ocean: response to seasonal and Late Quaternary surface water variability

The present study was initiated to ascertain the significance of coccolithophores as a proxy for paleoceanographic and paleoproductivity studies in the equatorial Atlantic. Data from a range of different samples, from the plankton, surface sediments as well as sediment cores are shown and compared with each other.In general, the living coccolithophores in the surface and subsurface waters show considerable variation in cell numbers and distribution patterns. Cell densities reached a maximum of up to 300×10³ coccospheres/l in the upwelling area of the equatorial Atlantic. Here, Emiliania huxleyi is the dominant species with relatively high cell numbers, whereas Umbellosphaera irregularis and Umbellosphaera tenuis are characteristic for oligotrophic surface waters. Although they are observed in high relative abundances, these species only occur in low absolute numbers. The lower photic zone is dominated by high abundances and considerable cell numbers of Florisphaera profunda.The geographical distribution pattern of coccoliths in surface sediments reflects the conditions of the overlying surface water masses. However, abundances of the oligotrophic species Umbellosphaera irregularis and Umbellosphaera tenuis are strongly diminished, causing an increase in relative abundance of the lower photic zone taxa Florisphaera profunda and Gladiolithus flabellatus.During the past 140,000 years the surface water circulation of the equatorial Atlantic has changed drastically, as can be seen from changes in the coccolithophore species composition, absolute coccolith numbers, as well as coccolith accumulation rates. Significant increases in coccolith numbers and accumulation rates is observed in the southern equatorial Atlantic during the last glacial interval (oxygen isotope stages 2–4), which we attribute to enhanced upwelling intensities and advection of cool nutrient rich waters at this site. In the western equatorial Atlantic we observe an opposite trend with decreasing numbers of coccoliths during glacial periods, which probably is caused by a deepening of the thermocline.     

The Late Glacial–Holocene record from Central Adriatic Sea: Paleoenvironmental reconstruction based on benthic foraminiferal assemblages

New micropaleontological data coming from three cores collected on the meso-Adriatic continental shelf (Vasto area) are studied. Comparisons to foraminiferal assemblages and radiocarbon dates previously collected from cores in the San Benedetto del Tronto and Tremiti areas allow the correlation of patterns observed in shallow water areas with those in deeper parts of the basin. We focused on the response of benthic foraminifera during the Holocene high-stand, corresponding to the installation of the recent sedimentary and trophic system. An influence of anthropogenic impacts cannot be ruled out; its effects consist of a depletion of oxygen level with a consequent modification of the structure of benthic foraminiferal assemblages. During the glacial/post glacial cycle, three phases, characterized by a total of six foraminiferal biofacies including different species assemblages were recognized. The first phase, from 14 kyr BP to 11 kyr BP, corresponds to the Bölling/Alleröd and Younger Dryas cold event, before the Holocene sea-level rise. During this phase, the continental shelf was characterized by an infralittoral environment with productive waters owing to the proximity of the Po river delta at the edge of the Mid-Adriatic Deep. The second phase, from 11 kyr BP to 4 kyr BP, represents the Holocene sea-level rise and is characterized by a condensed sedimentation spread over the entire basin. The third phase corresponds to the Holocene high-stand, during which time the modern current system became established. During this phase, the eastern portion of the shelf underwent to the central part of the mud-belt, corresponding to the sub-recent conditions. Recent eutrophication resulting from human activities over the last few centuries is evidenced by frequency fluctuations of typically opportunistic taxa such as Nonionella turgida and Epistominella vitrea.     

Abyssal benthic foraminifera from the northwestern Pacific (Shatsky Rise) during the last 298 kyr

Benthic foraminifera in a gravity core from Shatsky Rise (northwestern Pacific, water depth 2612 m) show large fluctuations in accumulation rate, species composition and diversity over the last 298 kyr. The most important fluctuations (explaining more than 90% of the faunal variance) result from variations in relative abundance of the three most abundant species: Epistominella exigua, Alabaminella weddellensis and Uvigerina peregrina. High accumulation rates of U. peregrina, a species linked to high, continuous productivity, occurred with a 100 kyr periodicity at the end of glacial stages, and correspond to high mass accumulation rates of organic carbon. Peak accumulation rates of E. exigua occurred during glacial stage 4 and the middle part of glacial stages 8 and 6, whereas A. weddellensis was dominant in the early part of stages 8 and 6, and the late part of stage 3. A high abundance of these species probably indicates lower overall productivity than a high abundance of U. peregrina, and their relative and absolute abundances may be linked not simply to the amount of organic matter delivered to the sea floor, but to the intermittent delivery of fresh, easily degraded organic matter. The overall species diversity is negatively correlated to the relative abundance of E. exigua, but not to that of A. weddellensis, implying that these species have different environmental preferences, although both have been linked to periodic phytodetritus deposition in the present oceans.