Foraminifera Globigerina Bulloides Research Articles

Surface oceanographic changes from ∼ 25,000 to 3500 cal yr BP in the eastern Arabian Sea

Multiproxy data of pteropods and planktic foraminifera from Core SK291/GC17 suggest significant surface paleoceanographic shifts from ∼25,000 to 3500 calibrated years before present (cal yr BP) in the eastern Arabian Sea (EAS). Increased values of the global ice-volume free stable oxygen isotope ratio of seawater (δ18Osw IVF) and stable oxygen isotope ratio in pteropod (δ18OH. inflatus IVF), as well as lower percentages of planktic foraminifer Globigerina bulloides from ∼22,000 to 18,000 cal yr BP, correspond to the weak Indian summer monsoon (ISM) during the Last Glacial Maximum (LGM). In the early Holocene, the EAS witnessed an increase in upwelling driven by strong summer monsoon winds, and a decrease in salinity corresponding with enhanced precipitation in the Western Ghats region. From ∼4800 to 4000 cal yr BP, including the 4.2 ka event, a prominent increase in the δ18OH. inflatus IVF values suggest an enhanced intrusion of the Arabian Sea High Saline Water (ASHSW) to the relatively deeper part of the mixed layer. In the same interval, the low representations of G. bulloides and thermocline dwelling planktic foraminifera indicate weakening of the coastal upwelling and a reduction in surface productivity due to weak ISM across the Arabian Sea, with more severe conditions in the EAS.

Large-scale culturing of the subpolar foraminifera Globigerina bulloides reveals tolerance to a large range of environmental parameters associated to different life-strategies and an extended lifespan.

The subtropical to subpolar planktic foraminifera Globigerina bulloides is a calcifying marine protist, and one of the dominant foraminiferal species of the Nordic Seas. Previously, the relative abundance and shell geochemistry of fossil G. bulloides have been studied for palaeoceanographic reconstructions. There is however a lack of biological observations on the species and a poor understanding of its ecological tolerances, especially for high latitude genotypes. Here, we present observations from the first extensive culturing of G. bulloides under subpolar conditions, including the first low temperature (6-13°C) and variable salinity (30-38) experiments. Carbonate chemistry (pH and [CO3 2-]) was also manipulated. Experimental conditions were chosen to reflect a range of plausible past and future scenarios for the Nordic Seas. We found G. bulloides to be tolerant of environmental conditions well outside their optimal range (<10°C, salinity <33, pH <8). Observed life span was up to three months, which was attributed to a microalgal diet. Two alternative life strategies were employed, whereby individuals either experienced rapid growth and death, or a prolonged lifespan with minimal growth and death via slow decay. We posit this could help explain differences in geochemical signals recorded from different size fractions of fossil specimens used for palaeoceanographic reconstructions.

Development of a Deep‐Water Carbonate Ion Concentration Proxy Based on Preservation of Planktonic Foraminifera Shells Quantified by X‐Ray CT Scanning

AbstractThe quantitative and objective characterization of dissolution intensity in fossil planktonic foraminiferal shells could be used to reconstruct past changes in bottom water carbonate ion concentration. Among proxies measuring the degree of dissolution of planktonic foraminiferal shells, X‐ray micro‐Computed Tomography (CT) based characterization of apparent shell density appears to have good potential to facilitate quantitative reconstruction of carbonate chemistry. However, unlike the well‐established benthic foraminiferal B/Ca ratio‐based proxy, only a regional calibration of the CT‐based proxy exists based on a limited number of data points covering mainly low‐saturation state waters. Here we determined by CT‐based proxy the shell dissolution intensity of planktonic foraminifera Globigerina bulloides, Globorotalia inflata, Globigerinoides ruber, and Trilobatus sacculifer from a collection of core top samples in the Southern Atlantic covering higher saturation states and assessed the reliability of CT‐based proxy. We observed that the CT‐based proxy is generally controlled by deep‐water Δ[] like the B/Ca proxy, but its effective range of Δ[] is between −20 and 10 µmolkg−1. In this range, the CT‐based proxy appears directly and strongly related to deep‐water Δ[], whereas we note that in some settings, there appears to be a secondary influence on B/Ca which we suggest may be due to elevated alkalinity from carbonate dissolution in sediments. On the other hand, the CT‐based proxy is affected by supralysoclinal dissolution in areas with high productivity. Like the B/Ca proxy, the CT‐based proxy requires species‐specific calibration, but the effect of species‐specific shell difference in susceptibility to dissolution on the proxy is small.

ENSO vs glacial-interglacial-induced changes in the Kuroshio-Oyashio transition zone during the Pleistocene

The subarctic front (SAF) in the pelagic North Pacific is the northernmost front of the Kuroshio-Oyashio transition zone separating the subpolar and subtropical gyres and is marked by a strong sea surface temperature gradient. A complex interplay of e.g. variations of currents, the wind system and other forcing mechanisms causes shifts of the SAF’s position on timescales from orbital to interannual. In this study, we present proxy data from the Emperor Seamount chain, which reveal a link between long-term ENSO (El Niño/Southern Oscillation) dynamics in the tropics and shifts of the SAF. Based on sediment core SO264-45-2 from Jimmu Seamount (46°33.792’N, 169°36.072’E) located close to the modern position of the SAF, we reconstruct changes in (sub)surface temperature ((sub)SSTMg/Ca) and δ18Osw-ivc (approximating salinities) via combined Mg/Ca and δ18O analyses of the shallow-dwelling foraminifera Globigerina bulloides and the near-thermocline-dwelling Neogloboquadrina pachyderma, biological productivity (XRF-based Ba/Ti ratios), and terrigenous input via dust (XRF-based Fe). From ~600 to ~280 ka BP we observe significantly higher SSTMg/Ca than after an abrupt change at 280 ka BP. We assume that during this time warmer water from the Kuroshio-Oyashio transition zone reached the core site, reflecting a shift of the SAF from a position at or even north of our study site prior to 280 ka BP to a position south of our study site after 280 ka BP. We propose that such a northward displacement of the SAF between 600-280 ka BP was induced by sustained La Niña-like conditions, which led to increased transport of tropical ocean heat into the Kuroshio-Oyashio transition zone via the Kuroshio Current. After ~280 ka BP, the change to more El Niño-like conditions led to less heat transfer via the Kuroshio Current with the SAF remaining south of the core location. In contrast, our productivity record shows a clear glacial-interglacial pattern that is common in the North Pacific. We assume that this pattern is connected to changes in nutrient supply or utilization, which are not primarily driven by changes of the Kuroshio and Oyashio Currents or the SAF.

Indian monsoon variability during the past ∼8.5 cal kyr as recorded in the sediments of the northeastern Arabian Sea

We analyzed benthic and planktic foraminifera, total organic carbon and stable isotopes ratios of carbon and oxygen from 100 samples of marine sediment Core ABP, 25/03, off Gujarat in the northeastern Arabian Sea. Factor and cluster analyses of thirty highest-ranked benthic foraminifer species enabled to identify seven biofacies in the study sequence characterizing major changes in deep-sea environments during the past ∼8.5 cal kyr. The benthic foraminiferal data has been combined with population abundances of upwelling indicator planktic foraminifer Globigerina bulloides and mixed layer species from Core ABP, 25/03 and compared with that from ODP Hole 723A and Core ABP, 25/02, western and eastern Arabian Sea, respectively. A major change in benthic foraminifera occurs during ∼8.5 to 6 cal kyr BP, when the summer monsoon was stronger whereas during 6 to 2 cal kyr BP, the data suggest a weak summer monsoon. Planktic foraminifer Globigerina bulloides does not show any significant trend in the study core, but deep thermocline species Neogloboquadrina dutertrei and mixed layer species show significant trend with an inverse correlation with the TOC. This core records high TOC from ∼6.5 to 4 cal kyr BP and during the last 2 cal kyr and low TOC during ∼8.5 to 6.5 cal kyr BP and 4 to 2 cal kyr BP. It is interesting to note intervals of high mixed layer species coincide with high Neogloboquadrina dutertrei percentages and low TOC values at Core ABP, 25/03 during 8.5 to 6.5 cal kyr BP.The faunal data is supported by carbon and oxygen isotope ratios (δ13C and δ18O) of shells of benthic foraminifer Cibicides spp. to strengthen our understanding of changes in the eastern Arabian Sea since ∼8.5 cal kyr BP.

Abrupt intrinsic and extrinsic responses of southwestern Iberian vegetation to millennial‐scale variability over the past 28 ka

ABSTRACTWe present new high‐resolution pollen records combined with palaeoceanographic proxies from the same samples in deep‐sea cores SHAK06‐5K and MD01‐2444 on the southwestern Iberian Margin, documenting regional vegetation responses to orbital and millennial‐scale climate changes over the last 28 ka. The chronology of these records is based on high‐resolution radiocarbon dates of monospecific samples of the planktonic foraminifera Globigerina bulloides, measured from SHAK06‐5K and MD01‐2444 and aligned using an automated stratigraphical alignment method. Changes in temperate and steppe vegetation during Marine Isotope Stage 2 are closely coupled with sea surface temperature (SST) and global ice‐volume changes. The peak expansion of thermophilous woodland between ~10.1 and 8.4 cal ka bp lags behind the boreal summer insolation maximum by ~2 ka, possibly arising from residual high‐latitude ice‐sheets into the Holocene. Rapid changes in pollen percentages are coeval with abrupt transitions in SSTs, precipitation and winter temperature at the onset and end of Heinrich Stadial 2, the ice‐rafted debris event and end of Heinrich Stadial 1, and the onset of the Younger Dryas, suggesting extrinsically forced southwestern Iberian ecosystem changes by abrupt North Atlantic climate events. In contrast, the abrupt decline in thermophilous elements at ~7.8 cal ka bp indicates an intrinsically mediated abrupt vegetation response to the gradually declining boreal insolation, potentially resulting from the crossing of a seasonality of precipitation threshold.

Export flux succession of dinoflagellate cysts and planktonic foraminifera in an active upwelling cell off Cape Blanc (NW Africa)

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.

Reconstruction of the Persian Gulf SST variability over the last five millennia

Sea surface temperature (SST) variation in the Persian Gulf (PG) is investigated based on Mg/Ca ratio measured in the planktonic foraminifera Globigerina bulloides tests from a sediment core spanning the past 5000 years recovered from west of the Strait of Hormoz. The Mg/Ca SST data indicate a 3 °C fluctuation range with an overall temperature increase trend since the mid-Holocene. The average SST of the PG in the intervals of 5–4.2, 4.2–3, 3–1.8, 1.8–1 and 1 cal kyr BP to present, was 26.1, 26.8, 25.8, 26.3 and 26.7 °C, respectively. SST increase trend in the PG is attributed to seasonally opposing insolation changes led to northern hemisphere extra-tropics cooling and tropics warming. The short spells of decreased (increased) SST indicate a high correlation with an increase (decrease) in terrigenous silt deposited in the PG, representing humid (dry) climate conditions in the Middle East. Likely, northward migration of the Inter-tropical convergence zone during the warmer episodes of the mid to late Holocene results in a change to a drier climatic regime in S Iran. Although Arabian Sea SST has a considerable role in SST variations in the PG, however, SST of this landlocked water body is mainly controlled by a continental climate.

Early deglacial CO2 release from the Sub-Antarctic Atlantic and Pacific oceans

Over the last deglaciation there were two transient intervals of pronounced atmospheric CO2 rise; Heinrich Stadial 1 (17.5-15 kyr) and the Younger Dryas (12.9-11.5 kyr). Leading hypotheses accounting for the increased accumulation of CO2 in the atmosphere at these times invoke deep ocean carbon being released from the Southern Ocean and an associated decline in the global efficiency of the biological carbon pump. Here we present new deglacial surface seawater pH and CO2sw records from the Sub-Antarctic regions of the Atlantic and Pacific oceans using boron isotopes measured on the planktic foraminifera Globigerina bulloides. These new data support the hypothesis that upwelling of carbon-rich water in the Sub-Antarctic occurred during Heinrich Stadial 1, and contributed to the initial increase in atmospheric CO2. The increase in CO2sw is coeval with a decline in biological productivity at both the Sub-Antarctic Atlantic and Pacific sites. However, there is no evidence for a significant outgassing of deep ocean carbon from the Sub-Antarctic during the rest of the deglacial, including the second period of atmospheric CO2 rise coeval with the Younger Dryas. This suggests that the second rapid increase in atmospheric CO2 is driven by processes operating elsewhere in the Southern Ocean, or another region.

Limited variability in the phytoplankton Emiliania huxleyi since the pre-industrial era in the Subantarctic Southern Ocean

The Southern Ocean is warming faster than the average global ocean and is particularly vulnerable to ocean acidification due to its low temperatures and moderate alkalinity. Coccolithophores are the most productive calcifying phytoplankton and an important component of Southern Ocean ecosystems. Laboratory observations on the most abundant coccolithophore, Emiliania huxleyi, suggest that this species is susceptible to variations in seawater carbonate chemistry, with consequent impacts in the carbon cycle. Whether anthropogenic environmental change during the industrial era has modified coccolithophore populations in the Southern Ocean, however, remains uncertain. This study analysed the coccolithophore assemblage composition and morphometric parameters of E. huxleyi coccoliths of a suite of Holocene-aged sediment samples from south of Tasmania. The analysis suggests that dissolution diminished the mass and length of E. huxleyi coccoliths in the sediments, but the thickness of the coccoliths was decoupled from dissolution allowing direct comparison of samples with different degree of preservation. The latitudinal distribution pattern of coccolith thickness mirrors the latitudinal environmental gradient in the surface layer, highlighting the importance of the geographic distribution of E. huxleyi morphotypes on the control of coccolith morphometrics. Additionally, comparison of the E. huxleyi coccolith assemblages in the sediments with those of annual subantarctic sediment trap records found that modern E. huxleyi coccoliths are ∼2% thinner than those from the pre-industrial era. The subtle variation in coccolith thickness contrasts sharply with earlier work that documented a pronounced reduction in shell calcification and consequent shell-weight decrease of ∼30-35% on the planktonic foraminifera Globigerina bulloides induced by ocean acidification. Results of this study underscore the varying sensitivity of different marine calcifying plankton groups to ongoing environmental change.

Surface and deep water variability in the Western Mediterranean (ODP Site 975) during insolation cycle 74: High-resolution calcareous plankton and molecular biomarker signals

We reconstructed changes in productivity and surface/subsurface and deep-water dynamics in the Western Mediterranean through a multi-proxy study of Ocean Drilling Program Site 975 between late Marine Isotope Stage (MIS) 20 and early interglacial MIS 19. Our high-resolution study (down to ~200-year resolution) combines calcareous plankton assemblages (coccolithophores and foraminifera), biomarkers (C37-alkenones, n-alkanes, n-alcohols) and elemental proxies (total organic carbon, total nitrogen, calcium carbonate). Surface water conditions are derived (i) from high-resolution δ18O and δ13C records obtained from the planktonic foraminifer Globigerina bulloides, and (ii) from summer and winter, foraminifera-based sea surface temperature reconstructions (SSTJAS-foram, SSTJFM-foram) achieved through transfer function. The integration of the whole dataset makes it possible to identify in the Balearic Sea, and to accurately characterize for the first time, an Organic Rich Layer (ORL) during latest MIS 20-early MIS 19, close to i-cycle 74. Its presence is marked firstly by higher values of total nitrogen (TN) and an increase of total C37-alkenone and total organic carbon (TOC) preserved in the sediments. The multi-proxy approach reveals that the deglacial phase played a prominent role for ORL formation that was characterized by centennial scale phases. The alcohol preservation index (API) suggests that the shoaling of the circulation, which boosted marine productivity, started in the deglaciation and, in combination with freshening by Atlantic water inflow/riverine input and surface water buoyancy during sea level rising, culminated during the ORL event. At this time calcareous plankton proliferated on subsurface-surface waters, benefiting from ameliorating conditions, which promoted maximum marine productivity and higher organic matter preservation on the seafloor.

Onset and demise of coral reefs, relationship with regional ocean circulation on the Wyville Thomson Ridge

Results from a vibrocore collected on the northern edge of the Scottish continental shelf at around 300 m water depth, on the Wyville Thomson Ridge, enable to reconstruct the history of cold water coral (CWC) reef growth and demise during the Holocene period. We report on significant age differences between U/Th and 14C dates obtained on pristine well-preserved CWC (Lophelia pertusa), which may reflect an diagenetic process that standard quality tests have failed to highlight. Additional 14C dates derived from bivalve fragments (Venus sp.) and foraminifera (Cibicides refulgens) samples show a gradual ageing with core depth but with significant age inversions in the lower section of the sediment core, which we consider reflects sedimentary mixing. We thus chose to derive an independent age model using planktonic foraminifera Globigerina bulloides stable isotope profiles. The vibrocore record is divided into 3 phases: 1- Mixed sediment deposits of glacial age corresponding to the base of the core with ages older than 13 cal ka BP, 2- The end of the deglacial/early Holocene between 13 and 9 cal ka BP and 3- Finally, the Holocene period from around 9 cal ka BP with abundant Lophelia pertusa fossils. Siliciclastic grain size and clay mineralogical composition show two significant shifts at around 13 and 9 cal ka BP indicating changes in sedimentary sources and transport associated with the dynamics and flow patterns of surface currents during the deglaciation and Holocene. Our results show that the onset of CWC reef growth on the Wyville Thomson Ridge occurred around 9 cal ka BP and was associated with a shift in flow patterns of surface currents in this area. This change of circulation patterns induced favourable sedimentological and hydrological conditions for corals to grow, and is associated with large scale modifications of North Atlantic circulation patterns at the end of the deglaciation.

Holocene hydrography evolution in the Alboran Sea: a multi-record and multi-proxy comparison

Abstract. A new high-resolution deglacial and Holocene sea surface temperature (SST) reconstruction is presented for the Alboran Sea (western Mediterranean), based on Mg∕Ca ratios measured in the planktonic foraminifera Globigerina bulloides. This new record is evaluated by comparison with other Mg∕Ca SST records and previously published alkenone SST reconstructions from the same region for both the Holocene and glacial periods. In all cases there is a high degree of coherence between the different Mg∕Ca SST records but strong discrepancies when compared to the alkenone SST records. We argue that these discrepancies are due to differences in the proxy response during deglaciation which we hypothesize to reflect a resilience strategy of G. bulloides, changing its main growth season, and consequently Mg∕Ca records a shorter deglacial warming than alkenones. In contrast, short-term Holocene SST variability is larger in the Mg∕Ca SST than in the alkenone SST records. We propose that the larger Mg∕Ca SST variability is a result of spring temperatures variability, while the smoothed alkenone SST variability represents averaged annual temperatures. The Mg∕Ca SST record differentiates the Holocene into three periods: (1) the warmest SST values occurred during the Early Holocene (11.7–9 cal. kyr BP), (2) a continuous cooling trend occurred during the Middle Holocene that culminated in the coldest Holocene SST having a double cold peak structure centred at around 4.2 cal. kyr BP, and (3) the Late Holocene (4.2 cal. kyr BP to present) did not follow any clear cooling/warming trend although millennial-scale oscillations were enhanced. This SST evolution is discussed in the context of the changing properties in the Atlantic inflow water associated with North Atlantic circulation conditions and also with local hydrographical and atmospheric changes. We propose that a tight link between North Atlantic circulation patterns and the inflow of surface waters into the Mediterranean played a major role in controlling Holocene climatic variability of this region.

Modern, Preindustrial, and Past (Last 25 ka) Carbon Isotopic (δ13C) Variability in the Surface Waters of the Southwest Pacific

AbstractCarbon stable isotopes (δ13C) in modern seawater samples and planktic foraminifera Globigerina bulloides from core top and downcore sediments are used to estimate the distribution of δ13C of dissolved inorganic carbon (DIC) in the surface waters of the southwest Pacific in the modern, preindustrial (PI), and over the last 25 kyr. The predicted δ13C distribution in the modern (δ13CDIC), PI (δ13CPI), and late Holocene (from planktic foraminifera Globigerina bulloides [temperature corrected δ13CG.bulloidesTC] from core tops) displays a broad peak at the subtropical front) and subantarctic surface waters due to the combination of high biological productivity and thermodynamic air‐sea gas exchange of CO2 in this region. The estimated δ13CPI values and measured δ13CG.bulloidesTC values from the core tops are higher than the modern values due to the Suess Effect. However, there is poor agreement between the δ13CPI values and core top δ13CG.bulloidesTC values south of 40°S as the back‐calculation approach using chlorofluorocarbon‐11 (CFC‐11) method for removing the anthropogenic δ13C is not effective at these higher southern latitudes. The δ13CG.bulloidesTC from a latitudinal transect of cores in the southwest Pacific were compiled by region using a Monte Carlo approach to determine the long‐term trends in δ13C over the last 25 kyr. Glacial subantarctic δ13CG.bulloidesTC values are low, while subtropical δ13C G.bulloidesTC are high. The peak in δ13CG.bulloidesTC values shifts south in the early Holocene. These latitudinal variations in δ13C G.bulloidesTC are linked to changes in ocean circulation, biological productivity (associated with the shifts in the subtropical front), and air‐sea CO2 exchange, likely related to the structure and position of the Southern Hemisphere Westerly Wind in the South Pacific region.

Dinoflagellate cyst paleoecology during the Pliocene–Pleistocene climatic transition in the North Atlantic

Dinoflagellate cysts (dinocysts) are widely used as tracers of sea surface conditions in late Quaternary marine records. However, paleoenvironmental reconstructions across the Pliocene–Pleistocene climatic transition and beyond are limited because the hydrologic conditions influencing assemblage compositions may not have a modern analogue, and the ecological optima of extinct dinocyst species are not well known. From a study of two cored sites in the central and eastern North Atlantic, we bypass these issues by statistically analyzing the variations in dinocyst assemblage composition and comparing the results directly to paleoecological parameters (δ18Obulloides, δ18Osalinity, and geochemical proxies for sea surface temperature [SST]) derived from the planktonic foraminifer Globigerina bulloides recovered from the same samples as the dinocysts. Through canonical correspondence analysis we demonstrate the co-variation of seasonality and dinocyst paleoproductivity. We show that Pyxidinopsis braboi is a cold tolerant species with an optimum SST between 12 and 14°C. We extend the use of Nematosphaeropsis labyrinthus as an indicator of transitional climatic conditions to the Pliocene, we offer evidence for the correlation of Bitectatodinium tepikiense and Filisphaera microornata to high seasonality, and we reiterate an apparent link between Spiniferites mirabilis and eastern North Atlantic water masses. Finally, we confirm that Habibacysta tectata is cold-tolerant rather than a strictly cold-water indicator, that Operculodinium? eirikianum is a cold-intolerant species favoring outer neritic environments, and that Ataxiodinium confusum and Invertocysta lacrymosa are both warm-water species.

Isotopic disequilibrium in Globigerina bulloides and carbon isotope response to productivity increase in Southern Ocean.

Oxygen and carbon isotope ratios in planktonic foraminifera Globigerina bulloides collected from tow samples along a transect from the equatorial Indian ocean to the Southern Ocean (45°E and 80°E and 10°N to 53°S) were analysed and compared with the equilibrium δ18O and δ13C values of calcite calculated using the temperature and isotopic composition of the water column. The results agree within ~0.25‰ for the region between 10°N and 40°S and 75–200 m water depth which is considered to be the habitat of Globigerina bulloides. Further south (from 40°S to 55°S), however, the measured δ18O and δ13C values are higher than the expected values by ~2‰ and ~1‰ respectively. These enrichments can be attributed to either a ‘vital effect’ or a higher calcification rate. An interesting pattern of increase in the δ13C(DIC) value of the surface water with latitude is observed between 35°S and~ 60°S, with a peak at~ 42°S. This can be caused by increased organic matter production and associated removal. A simple model accounting for the increase in the δ13C(DIC) values is proposed which fits well with the observed chlorophyll abundance as a function of latitude.

Increased seasonality during the intensification of Northern Hemisphere glaciation at the Pliocene–Pleistocene boundary ∼2.6 Ma

Near the Pliocene–Pleistocene (Neogene–Quaternary) boundary during Marine Isotope Stage (MIS) 104 at around 2.6 Ma, the intensification of Northern Hemisphere glaciation (iNHG) was marked by a southward shift of the North Atlantic Current and Arctic Front and a concurrent drop in sea surface temperature (SST) in the mid-latitudes of the North Atlantic. Here we evaluate two Mg/Ca-based sea-surface temperature reconstructions for the 2.78–2.52 Ma interval using the planktonic foraminifera Globigerina bulloides and Globigerinoides ruber (white) sensu stricto as the biotic carriers (SSTbul and SSTrub), and a reconstruction based on the alkenone saturation index (SSTalk). The Mg/Ca-based SST estimates begin to diverge from one another significantly during MIS G3 (∼2.66 Ma), culminating during MIS 104 (∼2.60 Ma), and remaining distinctly different for the remaining 80 kyr of the studied interval. The magnitude of this divergence (ΔSSTrub-bul) is caused mainly by a decrease in SSTbul, with SSTrub remaining relatively constant. We attribute this difference to a drop in SST during the early-spring blooming season of G. bulloides that was less pronounced during the late-summer blooming season of G. ruber. Using ΔSSTrub-bul as a seasonality indicator, we show that seasonality increased significantly owing to a drop in early spring temperatures from MIS 104 onwards, and peaked during glacial episodes. Maximum seasonality occurred during MIS 104 in coincidence with a major expansion of the circum-Atlantic ice sheets, particularly the North American ice sheet. This expansion appears to have had a critical influence on global climate and especially seasonality.

Evolution of the South Asian monsoon wind system since the late Middle Miocene

Abstract The evolution and timing of the inception of the South Asian monsoon (SAM) has been one of the most debated climatic mysteries amongst climate workers. The Himalaya–Tibetan plateau (HTP) complex has been implicated as the main driver for SAM circulation. However, there are intense debates as to whether there was a critical elevation of the HTP complex that drove the SAM and at what time such a critical elevation was attained. Also, the role of the HTP complex in driving the SAM has been questioned in recent climate model studies. The model simulation suggests a major intensification of the SAM as early as ~ 30 Ma, whereas marine records from the Arabian Sea indicate a major strengthening of the South Asian summer or southwest monsoon winds between 10 and 8 Ma. The continental vegetation, on the other hand, captured a major transition from C 3 to C 4 type during 8–7 Ma but whether this change was local or global and whether this transition was driven by monsoonal precipitation is not unanimously accepted. Our new record from Ocean Drilling Program holes located off the Oman margin and on the Owen Ridge, western Arabian Sea shows appearance of monsoon wind proxy planktic foraminifer Globigerina bulloides , a significant increase in total organic carbon, and a negative shift in stable carbon isotope record of benthic foraminifera at ~ 12.9 Ma. These proxies indicate that present day South Asian monsoon wind system began to develop during the late Middle Miocene (~ 12.9 Ma) and summer monsoon was in its full strength in the late Miocene (~ 7 Ma). From 11 to 7 Ma, the summer monsoon was weaker when winter monsoon was stronger.

Observation of the dissolution process of Globigerina bulloides tests (planktic foraminifera) by X‐ray microcomputed tomography

AbstractWe performed a 9 day dissolution experiment with tests of the planktic foraminifer Globigerina bulloides at pH 6.7 ± 0.1 in water undersaturated with respect to calcite. The initial stage of the dissolution process, which is not recognizable from the surface structure of the tests, was quantitatively evaluated by X‐ray microcomputed tomography (XMCT). XMCT revealed three distinct test structures: early‐developed calcite formed during the juvenile stage of G. bulloides, an inner calcite layer, and an outer calcite layer. The test ultrastructure was observed by scanning electron microscopy, and CT number evaluated the density distribution in the test. The early‐developed calcite and inner calcite layer had low CT numbers (500–1300; low density, porous) and were sensitive to dissolution, whereas the outer calcite layer had high CT numbers (&lt;1300; high density) and resisted dissolution. Both the modes and the frequencies of the CT numbers decreased with progress of dissolution. Changes in the CT number histogram with progress of dissolution were quantified in terms of the percentage of calcite volume accounted for by low‐density calcite (% Low‐CT‐number calcite). A clear linear relationship (R2 = 0.87) between % Low‐CT‐number calcite and % Test weight loss was found. This relationship indicates that the amount of test dissolution can be estimated from the distribution of CT numbers. We propose that XMCT measurements will be useful for quantitatively estimating the amount of carbonate loss from foraminiferal tests by dissolution.

Contribution of seasonal sub-Antarctic surface water variability to millennial-scale changes in atmospheric CO2 over the last deglaciation and Marine Isotope Stage 3

The Southern Ocean is thought to have played a key role in past atmospheric carbon dioxide (CO2,atm) changes. Three main factors are understood to control the Southern Ocean's influence on CO2,atm, via their impact on surface ocean pCO2 and therefore regional ocean–atmosphere CO2 fluxes: 1) the efficiency of air–sea gas exchange, which may be attenuated by seasonal- or annual sea-ice coverage or the development of a shallow pycnocline; 2) the supply of CO2-rich water masses from the sub-surface and the deep ocean, which is associated with turbulent mixing and surface buoyancy- and/or wind forcing; and 3) biological carbon fixation, which depends on nutrient availability and is therefore influenced by dust deposition and/or upwelling. In order to investigate the possible contributions of these processes to millennial-scale CO2,atm variations during the last glacial and deglacial periods, we make use of planktonic foraminifer census counts and stable oxygen- and carbon isotope measurements in the planktonic foraminifera Globigerina bulloides and Neogloboquadrina pachyderma (sinistral) from marine sediment core MD07-3076Q in the sub-Antarctic Atlantic. These data are interpreted on the basis of a comparison of core-top and modern seawater isotope data, which permits an assessment of the seasonal biases and geochemical controls on the stable isotopic compositions of G. bulloides and N. pachyderma (s.). Based on a comparison of our down-core results with similar data from the Southeast Atlantic (Cape Basin) we infer past basin-wide changes in the surface hydrography of the sub-Antarctic Atlantic. We find that millennial-scale rises in CO2,atm over the last 70 ka are consistently linked with evidence for increased spring upwelling, and enhanced summer air–sea exchange in the sub-Antarctic Atlantic. Parallel evidence for increased summer export production would suggest that seasonal changes in upwelling and air–sea exchange exerted a dominant influence on surface pCO2 in the sub-Antarctic Atlantic. These results underline the role of Southern Ocean dynamics, in particular their seasonal variations, in driving millennial-scale variations in CO2,atm.