Ruber Sensu Stricto Research Articles

Obliquity-driven subtropical forcing of the thermocline after 240 ka in the southern sector of the Western Pacific Warm Pool

Changes in vertical geochemical gradients within the Western Pacific Warm Pool (WPWP) are important indicators of the region's upper ocean response to climate changes and to the oceanographic coupling between the thermocline and surface mixed layer. Here, we reconstruct temperature and δ18Osw at International Ocean Discovery Program Site U1486 (1332 m water depth, 2°22′S, 144°36′E) located in the Bismarck Sea in the southern sector of the WPWP. A 670-kyr record of Δδ18O between the surface-dwelling foraminifera Globigerinoides ruber sensu stricto and the thermocline-dwelling foraminifera Pulleniatina obliquiloculata and Globorotalia tumida (when combined with Mg/Ca-based temperature and δ18Osw estimates) suggests long-term thermocline shoaling and a progressively increasing vertical salinity gradient commencing near 240 ka. Through a detailed comparison to other Pacific records, it becomes clear this is not solely a local phenomenon, as we identify widespread cooling of the thermocline in the low-latitude Pacific after ∼240 ka. After examining our temperature reconstructions alongside new δ18Osw and constant flux proxy-derived focusing factor records, we potentially also validate previous models which find obliquity-induced strengthening of low-latitude Pacific currents. We extend this to support periods of increased transport of high-salinity thermocline water masses through the South Pacific low-latitude western boundary current system. These results indicate greater variability in thermocline circulation given amplified obliquity and strengthen previous evidence that variability in the WPWP thermocline is independent from the drivers of WPWP surface variability.

Holocene sea surface temperature and salinity variations in the central South China Sea

The South China Sea (SCS) is sensitive to the East Asian monsoon (EAM) and changes in the Western Pacific. However, because of the difficulty of sampling and lack of data in the central SCS, many uncertainties remain the primary driving force of paleoclimatic variations. Here, a sediment core JL136, from the central SCS, was used to evaluate variations of sea surface temperature (SST) and salinity (SSS) during the Holocene, and then the corresponding influencing factors were analyzed, based on measured Mg/Ca ratio and stable isotopic compositions (δ18O and δ13C) of planktonic foraminifera Globigerinoides ruber sensu stricto (s.s.). The results show that the Mg/Ca-SST change is consistent with the speleothem δ18O records across most Asian monsoon regions on the Holocene long-term trend, revealing that the central SCS record is a response to the East Asian Summer Monsoon (EASM) intensity changes induced by Northern Hemisphere summer insolation (NHSI) variation. During the early Holocene (11.6–8.8 ka), SSS was low with an average of 32.7 psu, which may be related to palaeogeographic variations with sea level changes and the increased coastal water due to the opening of the Taiwan Strait. During the mid-late Holocene, high salinity ocean water was fully exchanged with the SCS due to the influence of high sea level, with an overall higher salinity in the SCS compared with the early Holocene (average 33.8 psu). In addition, the SSS broadly showed an increasing trend, which may be mainly controlled by the weakening EASM intensity and the decreasing precipitation during the low NHSI.

Millennial-scale variability of Indian summer monsoon constrained by the western Bay of Bengal sediments: Implication from geochemical proxies of sea surface salinity and river runoff

The Indian summer monsoon (ISM) is critical to billions of people living in the region. In this study, high-resolution major element composition and Globigerinoides ruber sensu stricto (G. ruber s.s.) δ18O measurements of a sediment core collected from the western Bay of Bengal indicated a high sensitivity of the study site to monsoonal climate variability related to climatic changes on a timescale of millennia (Younger Dryas and Heinrich Stadials) in the North Atlantic during the last 80 kyr. High G. ruber s.s. δ18O and ice volume-corrected ocean surface water δ18O and increased Ca/Ti ratios during Younger Dryas and Heinrich Stadials HS1–6 indicate the decreased summer monsoonal rainfall in the Indian peninsula during these periods. The increases of G. ruber s.s. δ18O records in the western Bay of Bengal during Younger Dryas and Heinrich Stadials were similar in previous records of G. ruber s.s. δ18O in the northern Bay of Bengal and stalactite δ18O in northern India. This implies that the decreased precipitation during these periods may have been induced by the southward migration of the northern boundary of Intertropical Convergence Zone (ITCZ) over the Indian peninsula during Younger Dryas and Heinrich Stadials. Previous records of δ18O values in the Andaman Sea located in the eastern Bay of Bengal showed no remarkable decrease of summer precipitation during HS1 and 5. These differences may be explained by the different migration trends of summer ITCZ position in the eastern Bay of Bengal versus western Bay of Bengal during HS1 and 5.

Paleoceanographic significance of Globigerinoides ruber (white) morphotypes from the Andaman Sea

Globigerinoides ruber (white), a tropical to sub-tropical planktonic foraminiferal species which dwells in the mixed layer, has garnered significant attention of paleoceanographic investigations. Recent studies on G. ruber (w) morphotypes (G. ruber sensu stricto (s.s.) and G. ruber sensu lato (s.l.)) have inferred differences in seasonal calcification depths and offsets in isotopic and elemental data. To decipher if the morphotype abundances vary in response to changes in Indian Summer Monsoon (ISM), two well-dated sediment cores from the Andaman Sea, a core monsoon area in the northeastern Indian Ocean, are investigated. The abundance variations of G. ruber s.s. are very similar to total G. ruber (w), whereas G. ruber s.l. is very sensitive to the hydrologic changes linked to the ISM variability. Increased abundances of G. ruber s.l. were observed during periods of weak stratification and the resultant increase in productivity. The last glacial maximum (LGM), deglacial, Younger Dryas (YD) and late Holocene periods with weak ISM are characterized by a high abundance of G. ruber s.l. Conversely, this morphotype abundance decreased from 30 to 23 ka, Bølling/Allerød (B/A), early and mid-Holocene when the ISM was enhanced. The large variation of G. ruber morphotypes in the Andaman Sea is attributed to the selective preference of nutrient-rich water which varied with water column changes (salinity-controlled stratification) associated with ISM evolution. The elevated abundances of G. ruber s.l. during the glacial period closely match with the available records from other regions, possibly making them potential climatic markers.

Sea surface temperature and salinity variations from the end of Last Glacial Maximum (LGM) to the Holocene in the northern South China Sea

Paleoclimatic and palaeoceanographic studies in land and sea, especially monsoon and sea surface conditions in the past, help to better understand the internal feedback processes within the climate system. However, asynchronisms were observed between records from land and sea at the warming transition from the end of the Last Glacial Maximum (LGM) to the Holocene, raising debating of primary forcing of paleoclimatically variations. In this work, a sediment core ZSQD2, from the northern South China Sea (SCS), spanning the end of the LGM to the mid-Holocene, was utilized to evaluate variations of sea surface temperature and salinity, and then analyze the correspond pattern to the Asian Monsoon system, based on measured Mg/Ca ratio and isotopic compositions (δ18O and δ13C) of planktonic foraminifera Globigerinoides ruber sensu stricto (s.s.). These records show that the warming started from ~16 ka, which agrees well with the East Asian Summer Monsoon (EASM) records, revealing the synchronism in variations between the northern SCS sea surface conditions and the palaeoclimate of China Mainland, at the beginning of warming transition from the end of LGM. Mirrored SST and SSS variations were in step with stalagmite δ18O signals, also unveiling a close relationship between sea surface water condition in the northern SCS and the EASM intensity. On millennial timescales, asynchronism between the China Mainland palaleoclimatic changes and seawater conditions of the northern SCS is displayed by declined SST and heavier δ13CG.ruber during ~15–13.5 and ~ 11.5–9.5 ka, when the EASM enhanced and SSS declined. This is resulted from strengthened the EASM driven upwelling cooled down the surface water and enhanced nutrient supply in the northern SCS, combined increasing cold and fresh water input from the Pearl River.

Millennial-scale variability of East Asian summer monsoon inferred from sea surface salinity in the northern East China Sea (ECS) and its impact on the Japan Sea during Marine Isotope Stage (MIS) 3

Color alternations in deep-sea sediment in the Japan Sea have been thought to be linked to millennial-scale variations in the East Asian summer monsoon (EASM), associated with the Dansgaard-Oeschger (D-O) cycles and Heinrich events in the high-latitude North Atlantic during Marine Isotope Stage 3 (MIS 3). In this study, we investigate the variability of sea surface salinity (SSS) in the northern East China Sea (ECS) to evaluate the EASM precipitation in South China and its linkage to the sediment color of the Japan Sea during MIS 3. High time resolution (< 100 years) SSS along with sea surface temperature (SST) records were reconstructed using paired Mg/Ca and the oxygen isotope of planktic foraminifera Globigerionoides ruber sensu stricto from core KR07-12 PC-01 recovered from the northern ECS. The results indicate that millennial-scale variability of the SSS is observed with the amplitude of ~ ± 1 during MIS 3. The variations in SSS are well correlated to D-O cycles and Heinrichs. The EASM precipitation decreases in association with the southward shift of the westerly jet in D-O stadials and Heinrichs, suggesting suppressed moisture convergence along the EASM front associated with weakened North Pacific subtropical high in response to the slow-down of the Atlantic Meridional Overturning Circulation. In a comparison between the SSS in the ECS and the color alternation in the Japan Sea, closely correlated variations between the two records in the interval 44–34 ka indicate that the SSS in the ECS plays a crucial role in regulating nutrient and salinity inflow into the Japan Sea. However, the linkage becomes ambiguous, especially after ~ 30 ka, when the sea level falls toward the level of the last glacial maximum. This shift is associated with changes in sediment facies, confirming that the underlying mechanism in regulating the sedimentary change in the Japan Sea depends on the sea level.

Monsoon-influenced variations in plankton community structure and upper-water column stratification in the western Bay of Bengal during the past 80 ky

To enhance our understanding of the forcing factors of marine biological community structure in the Bay of Bengal, we studied proxies for biogenic carbonate and silicate production in the western Bay of Bengal and compared them with proxies for upper-ocean stratification and strength of the Indian summer monsoon (ISM). Specifically, we investigated the record in a sediment core from the western Bay of Bengal that extended to approximately 80 ky before the present (BP). The records of Globigerinoides ruber sensu stricto δ18O minus Neogloboquadrina dutertrei δ18O (Δδ18Or–d) were used to investigate changes in upper-ocean stratification. We reconstructed variations in mass accumulation rates (MARs) of CaCO3 and biogenic silica (BSi) to discuss plankton community structure. Greater difference between Globigerinoides ruber sensu stricto δ18O and Neogloboquadrina dutertrei δ18O during the marine isotope stage (MIS) 1 and 5a indicate intervals with less saline and more stratified conditions. In contrast, the relatively small difference between both planktic foraminiferal δ18O during MIS 2 and 4 indicates diminished stratification during these periods. Elevated carbonate production during MIS 2 and 4, demonstrated by the increase in CaCO3 MAR, could have been driven by lower freshwater influx to the western Bay of Bengal resulting from lower ISM precipitation, and thus the increased mixing and upwelling of deep nitrate into the photic zone. The relative increase in BSi MAR during periods of high ISM precipitation and decreased salinity of surface waters suggests a limited recovery in biological siliceous production under interglacial conditions, possibly due to enhanced fluvial SiO4 delivery to the ocean. Fluctuations of Δδ18Or–d during MIS 3 indicate that freshwater stratification was moderate and insufficient to restrict upwelling. This might have induced moderate biogenic silica and carbonate productions during this period.

Geochemical characteristics from tests of four modern planktonic foraminiferal species in the Indonesian Throughflow region and their implications

Test geochemistry of planktonic foraminifera is an indispensable tool in reconstructing past ocean hydrological changes. It is essential to investigate region-specific implications of test geochemistry, although those established from other regions can be broadly applied. In this study, characteristics of δ18O and Mg/Ca from tests of four planktonic foraminiferal species, Globigerinoides ruber sensu stricto (s.s.), Globigerinoides sacculifer, Pulleniatina obliquiloculata and Neogloboquadrina dutertrei, from 60 core-top sediment samples retrieved from the Indonesian Throughflow (ITF) region were studied. These geochemical data were compared with modern hydrographic profiles in order to assess their relations and to investigate potential implications of test geochemical parameters in reconstructing past oceanographic change in the ITF region. Calcification depths of these four species were first estimated based on comparison between measured test δ18O and predicted calcite δ18O that was calculated from modern temperature and salinity. The results indicate that G. ruber s.s. and G. sacculifer calcify within the mixed-layer at 0–50 m and 20–75 m, respectively, whereas P. obliquiloculata and N. dutertrei calcify within the thermocline at around 75 to 125 m. A combined study of excess Mg/Ca (difference between measured and predicted Mg/Ca) and salinity suggests that salinity exerts a negligible impact on test Mg/Ca of these foraminiferal species in the ITF region. Comparison of test Mg/Ca-derived temperatures with temperature profiles of the upper 200 m of the water column from the seas of the ITF region also indicate calcification depths of these species, which match well with the above estimations using test δ18O. It further indicates that G. sacculifer may be more sensitive in reflecting changes in the depth of the mixed-layer, highlighting a potential use of Mg/Ca temperature difference between G. ruber s.s. and G. sacculifer in reconstructing the depth of the mixed-layer in the ITF region.

Variation in subsurface water temperature and its link to the Kuroshio Current in the Okinawa Trough during the last 38.5 kyr

An 820.4-cm long piston core (YK12-15 PC-01) was obtained from site OT-1B (28°07.15′N, 127°11.97′E, 1050 m water depth), located in the middle of the Okinawa Trough in the southern East China Sea (ECS). For core YK12-15 PC-01, oxygen and carbon isotopes (δ18O and δ13C) of planktic foraminifera Globigerinoides ruber sensu stricto (250–355 μm size fraction) and Globorotalia inflata (>250 μm size fraction) together with total organic carbon (TOC), nitrogen content (TN), and δ13C of organic matter were analyzed for the last 38.5 thousand years (kyr). The δ18O records of G. ruber (δ18Oruber) and G. inflata (δ18Oinflata) were used to estimate the temperature/salinity of the surface and subsurface waters, respectively. In addition, we estimated the vertical temperature gradient between the surface and the subsurface based on the difference in δ18O between G. ruber and G. inflata (Δδ18Oruber-inflata). The δ18Oinflata and Δδ18Oruber-inflata indicate millennial-scale variations in association with the oscillations in Antarctic temperatures during the Marine Isotope Stage (MIS) 3. These results suggest that variations in subsurface temperature and vertical temperature gradient in the upper water column in the ECS were closely related to the Atlantic bipolar seesaw during MIS 3. The subsurface temperature in the ECS was higher at the beginning of Greenland stadials when the Antarctic temperatures showed the lowest values. The warming of the subsurface is likely attributed to the stronger Kuroshio transport. High peaks in the TOC and TN records coincide with the low peaks in δ18Oinflata (high temperature), suggesting higher primary productivity, which is in good agreement with the interpretation of the intensified Kuroshio. However, the subsurface warming was not recognized during Herinch events, indicating different temperature responses in the upper water column from other stadials.

Solar-, monsoon- and Kuroshio-influenced thermocline depth and sea surface salinity in the southern Okinawa Trough during the past 17,300 years

Factors influencing millennial-scale variability in the thermocline depth (vertical mixing) and sea surface salinity (SSS) of the southern Okinawa Trough (OT) during the past 17,300 years were investigated based on foraminifer oxygen isotope records of the surface dweller Globigerinoides ruber sensu stricto and the thermocline dweller Pulleniatina obliquiloculata in the AMS 14C dated OKT-3 core. The thermocline depth is influenced by surface thermal buoyancy (heat) flux, in turn controlled by the annual mean insolation at 30°N and the strength of the East Asian winter monsoon (EAWM). Strong insolation and weak EAWM tend to increase buoyancy gain (decrease buoyancy loss), corresponding to shallow thermocline depths, and vice versa. Regional SSS is influenced by the global ice volume, the Kuroshio Current (KC), and vertical mixing. A deep thermocline coincides with a high SSS because strong vertical mixing brings more, saltier subsurface KC water to the surface, and vice versa. Local SSS (excluding the global ice volume effect) became lower in the northern OT than in the southern OT after ~9.2 ka, implying that Changjiang diluted water had stronger influence in the northern sector. SSS show no major changes during the Bolling/Allerod and Younger Dryas events, probably because the KC disturbed the North Atlantic signals. This argues against earlier interpretations of sea surface temperature records of this core. Wavelet and spectral analyses of the Δδ18OP-G (δ18O of P. obliquiloculata minus G. ruber s.s.) and δ18Olocal records display 1,540-, 1,480-, 1,050-, 860-, 640-, and 630-year periods. These are consistent with published evidence of a pervasive periodicity of 1,500 years in global climate as well as EAWM and KC signatures, and a fundamental solar periodicity of 1,000 years and intermediary derived periodicity of 700 years.

A Comparison of Mg/Ca Ratios in Globigerinoides ruber (white): sensu stricto versus a Mixture of Genotypes

Abstract: Mg/Ca ratios were measured in core AAS9/21 from the eastern Arabian Sea (EAS), on two sets of planktonic foraminifera, one with a mixture of Globigerinoides ruber genotypes (sensu stricto and sensu lato) and the second which contained only G. ruber sensu stricto, living in the top 30 m of the water column. During the Holocene, sea surface temperatures (SSTs) in set 1 are cooler in comparison to that of set 2. During the deglaciation, SSTs derived using both these sets are similar, probably due to increased upwelling during this period. The drawback in using undifferentiated G. ruber is that the relative proportion of each genotype varies through different time periods as seen in the EAS and may influence the temperature reconstructions.

Biotic and geochemical (δ18O, δ13C, Mg/Ca, Ba/Ca) responses of Globigerinoides ruber morphotypes to upper water column variations during the last deglaciation, Gulf of Mexico

Within the Gulf of Mexico (GOM), the Bryant Canyon is exceptional in that rapid sedimentation sustained by high rates of terrigenous sediment delivery and surface marine productivity make this basin an excellent recorder of paleoenvironmental and paleoclimatic conditions. We present a new 21-kyr record of sea surface temperature (SST) and local salinity changes from the NW GOM (core JPC-26) approximated from combined Globigerinoides ruber morphotype-specific δ18O and Mg/Ca, as well as Ba/Ca, which reflect the subtropical GOM hydrographic dynamics and their relationship to both Mississippi River discharges and climate evolution during the last deglaciation. Overall, the reconstructed SST and SSS-related patterns reveal notable variations in amplitude between the analyzed morphotypes, adding valuable insights to previously published G. ruber (w, mixed) GOM records. Especially during the deglaciation, Bryant Canyon meltwater flooding events (BCDFs) and associated sea surface freshening seem to be more pronounced than the SST reduction. Our Mg/Ca-derived SST records of both morphotypes (G. ruber sensu stricto (s.s.) and G. ruber sensu lato (s.l.)) show comparable general trends, but with important SST differences (ΔT). We interpret down-core ΔT as a record of changing upper water column hydrography, with particular influence from the deglacial meltwater (BCDFs) or Holocene (BCHFs) flooding events. During the warm intervals, the deeper, thicker and probably more seasonally persistent mixed layer led to more uniform thermal conditions for both G. ruber morphotypes and therefore to a minimum ΔT. On the contrary, during the cold and low salinity periods, the shallower mixed layer favored more habitat divergence, and caused a maximum thermal gradient in the well-stratified upper water column. Overall, this supports the notion that G. ruber s.s. is consistently calcifying in warmer waters than G. ruber s.l., either due to a shallower depth habitat or to a more summer-weighted seasonal distribution. Moreover, intra-specific paired δ13C and Ba/Ca differences show that G. ruber s.l. is more sensitive to river water influence than G. ruber s.s., due to its ability to change its depth habitat and therefore exploit optimal (temperature, salinity, productivity, stratification) conditions during the low-salinity events. Overall, our observations support the current practice of treating the two G. ruber morphotypes separately and further illustrate the necessity to map both their spatial and temporal distribution.

Reconstructing the thermal structure of the upper ocean: Insights from planktic foraminifera shell chemistry and alkenones in modern sediments of the tropical eastern Indian Ocean

Shell chemistry of planktic foraminifera and the alkenone unsaturation index in 69 surface sediment samples in the tropical eastern Indian Ocean off West and South Indonesia were studied. Results were compared to modern hydrographic data in order to assess how modern environmental conditions are preserved in sedimentary record, and to determine the best possible proxies to reconstruct seasonality, thermal gradient and upper water column characteristics in this part of the world ocean. Our results imply that alkenone‐derived temperatures record annual mean temperatures in the study area. However, this finding might be an artifact due to the temperature limitation of this proxy above 28°C. Combined study of shell stable oxygen isotope and Mg/Ca ratio of planktic foraminifera suggests that Globigerinoides ruber sensu stricto (s.s.), G. ruber sensu lato (s.l.), and G. sacculifer calcify within the mixed‐layer between 20 m and 50 m, whereas Globigerina bulloides records mixed‐layer conditions at ∼50 m depth during boreal summer. Mean calcifications of Pulleniatina obliquiloculata, Neogloboquadrina dutertrei, and Globorotalia tumida occur at the top of the thermocline during boreal summer, at ∼75 m, 75–100 m, and 100 m, respectively. Shell Mg/Ca ratios of all species show a significant correlation with temperature at their apparent calcification depths and validate the application of previously published temperature calibrations, except for G. tumida that requires a regional Mg/Ca‐temperature calibration (Mg/Ca = 0.41 exp (0.068*T)). We show that the difference in Mg/Ca‐temperatures of the mixed‐layer species and the thermocline species, particularly between G. ruber s.s. (or s.l.) and P. obliquiloculata, can be applied to track changes in the upper water column stratification. Our results provide critical tools for reconstructing past changes in the hydrography of the study area and their relation to monsoon, El Niño‐Southern Oscillation, and the Indian Ocean Dipole Mode.

Tracking the equatorial front in the eastern equatorial Pacific Ocean by the isotopic and faunal composition of planktonic foraminifera

Abstract Core-top samples from the eastern tropical Pacific (10°N to 20°S) were used to test whether the ratio between Globorotalia menardii cultrata and Neogloboquadrina dutertrei abundance (Rc/d) and the oxygen isotope composition (δ18O) of planktonic foraminifera can be used as proxies for the latitudinal position of the Equatorial Front. Specifically, this study compares the δ18O values of eight species of planktonic foraminifera (Globigerinoides ruber sensu stricto (ss) and sensu lato (sl), Globigerinoides sacculifer, Globigerinoides triloba, Pulleniatina obliquiloculata, Neogloboquadrina dutertrei, Globorotalia menardii menardii, Globorotalia menardii cultrata and Globorotalia tumida) with the seasonal hydrography of the region, and evaluates the application of each species or combination of species for paleoceanographic reconstructions. The results are consistent with sea surface temperature and water column stratification patterns. We found that in samples north of 1°N, the Rc/d values tend to be higher and δ18O values of G. ruber, G. sacculifer, G. triloba, P. obliquiloculata, N. dutertrei, and G. menardii cultrata tend to be lower than those from samples located south of 1°N. We suggest that the combined use of Rc/d and the δ18O difference between G. ruber and G. tumida or between P. obliquiloculata and G. tumida are the most suitable tools for reconstructing changes in the latitudinal position of the Equatorial Front and changes in the thermal stratification of the upper water column in the eastern tropical Pacific.

A revised taxonomic and phylogenetic concept for the planktonic foraminifer species Globigerinoides ruber based on molecular and morphometric evidence

article i nfo Recent SSU rDNA sequence data of the paleoceanographically important planktonic foraminifer species Globigerinoides ruber showed the presence of two distinct phylogenetic lineages within this morphotaxon. The first lineage (G. ruber sensu stricto) includes the genetic type that corresponds to pink-pigmented G. ruber, as well as three of the five genetic types recognised in individuals of white G. ruber (labelled Type Ia, Type Ib and Type Ib2). The remaining two genetic types of G. ruber (white), labelled as Types IIa and IIb, represent a distinct phylogenetic lineage (G. ruber sensu lato), closer related to Globigerinoides conglobatus. Here we combine molecular clock and morphometric analyses to shed light on the taxonomical and phylogenetic significance of the presence of these two distinct lineages within the morphotaxon G. ruber .A molecular clock approach suggests a rather recent origin of the G. ruber sensu stricto lineage in the late Miocene and a split between G. ruber (pink) and the white genotypes Ia, Ib and Ib2 around 6 Ma. This indicates that all records of G. ruber prior to the G. ruber pseudo-extinction event at 8 Ma refer to an unrelated species (Globigerinoides subquadratus) and that the G. ruber (pink) lineage is substantially older than the first appearance of the pink pigmentation in the fossil record. In order to establish the taxonomic identity of the G. ruber sensu lato phylogenetic lineage, we conducted morphometric measurements on (i) pictures of specimens with known genetic identity, (ii) shells from sediment samples, (iii) specimens assigned to G. ruber and Globigerinoides elongatus from a museum collection and (iv) pictures of G. ruber sensu lato morphotype from recent literature. Our results suggest that specimens of Type IIa that represent the G. ruber sensu lato lineage are morphologically identical to the concept of the G. ruber sensu lato morphotype in recent literature, and that these morphotypes are consistent with the species definition of Globigerinoides elongatus. We therefore propose that the name G. elongatus (sensu d'Orbigny) should be reinstated and used for the genetic Type IIa. The name G. ruber (sensu d'Orbigny) should be reserved for specimens of the pink chromotype. Specimens of Types Ia, Ib and Ib2 require new species names, but our data are not sufficient to provide a morphological character separating these species from their sister G. ruber (pink), other than by their shell colouration.

Mg/Ca variability of the planktonic foraminifera G. ruber s.s. and N. dutertrei from shallow and deep cores determined by electron microprobe image mapping

The Mg/Ca ratio of planktonic foraminifera is widely used for reconstructing sea surface and thermocline temperatures. Dissolution, driven by changes in the bottom water calcite saturation state (a function of water mass chemistry and depth/pressure), lowers the bulk Mg/Ca ratio which results in an implied cooler reconstructed temperature. Here we employ electron microprobe analysis (EMPA) to characterize how dissolution alters shell Mg/Ca variability. Using EMPA we have generated Mg/Ca image maps of two species widely used in paleoceanographic reconstructions: Globogerinoides ruber sensu stricto (G. ruber s.s.) and Neogloboquadrina dutertrei (N. dutertrei). We first characterize the Mg/Ca ratio in the better-preserved shells from a shallow core (2.8 km) and compare those characteristics to the samples from a deep core (4.0 km) obtained from the same locality (Ceara Rise, western equatorial Atlantic). Though preliminary, results suggest EMPA Mg/Ca ratio mapping can reveal characteristics of well preserved shells that can be used to assess changes in the Mg/Ca ratio that occur with dissolution.

Low‐latitude control on seasonal and interannual changes in planktonic foraminiferal flux and shell geochemistry off south Java: A sediment trap study

Results from sediment trap experiments conducted in the seasonal upwelling area off south Java from November 2000 until July 2003 revealed significant monsoon‐, El Niño–Southern Oscillation–, and Indian Ocean Dipole–induced seasonal and interannual variations in flux and shell geochemistry of planktonic foraminifera. Surface net primary production rates together with total and species‐specific planktonic foraminiferal flux rates were highest during the SE monsoon‐induced coastal upwelling period from July to October, with three species Globigerina bulloides, Neogloboquadrina pachyderma dex., and Globigerinita glutinata contributing to 40% of the total foraminiferal flux. Shell stable oxygen isotopes (δ18O) and Mg/Ca data of Globigerinoides ruber sensu stricto (s.s.), G. ruber sensu lato (s.l.), Neogloboquadrina dutertrei, Pulleniatina obliquiloculata, and Globorotalia menardii in the sediment trap time series recorded surface and subsurface conditions. We infer habitats of 0–30 m for G. ruber at the mixed layer depth, 60–80 m (60–90 m) for P. obliquiloculata (N. dutertrei) at the upper thermocline depth, and 90–110 m (100–150 m) for G. menardii in the 355–500 μm (&gt;500 μm) size fraction corresponding to the (lower) thermocline depth in the study area. Shell Mg/Ca ratio of G. ruber (s.l. and s.s.) reveals an exponential relationship with temperature that agrees with published relationships particularly with the Anand et al. (2003) equations. Flux‐weighted foraminiferal data in sediment trap are consistent with average values in surface sediment samples off SW Indonesia. This consistency confirms the excellent potential of these proxies for reconstructing past environmental conditions in this part of the ocean realm.

The occurrence of two genotypes of the planktonic foraminifer Globigerinoides ruber (white) and paleo-environmental implications

Planktonic foraminifera provide a record of the upper ocean environment in the chemical and isotopic composition of individual shells. Globigerinoides ruber is a common tropical–subtropical planktonic foraminifer, and this species is used extensively for reconstruction of the paleo-environment. The different stable isotopic compositions of two morphotypes, G. ruber sensu stricto (s.s.) and G. ruber sensu lato (s.l.), first identified in sediments, suggested that G. ruber s.s. was dwelling in the upper 30 m of the water column and G. ruber s.l. at greater depths. Plankton tows and sediment trap experiments provided additional evidence distinguishing the two morphotypes and their habitats and invited the question as to whether the two morphotypes could be distinguished genetically. In this study, using phylogenetic analysis of nuclear partial small subunit ribosomal DNA (SSU rDNA) gene sequences representing 12 new and 16 known sequences, we identified four genotypes within G. ruber white variation; one of which is a sister group of Globigerinoides conglobatus, whereas the three others were sister groups of the G. ruber pink variation. Moreover, these two major groups corresponded to morphological differences described as G. ruber s.l. and s.s., respectively. This genetic evidence corroborates differences between the two morphotypes in the isotope record, and it will contribute to a more precise reconstruction of the thermal structure of the water column.

Proxy dependence of the temporal pattern of deglacial warming in the tropical South China Sea: toward resolving seasonality

Sea surface temperatures (SSTs), reconstructed from two Globigerinoides ruber (white) morphotypes (G. ruber sensu stricto, (s.s.); G. ruber sensu lato, ( s.l.)) Mg/Ca and alkenones (UK′37) from core MD01-2390 from the tropical South China Sea (SCS) during the last deglaciation reveal a proxy-dependent discrepancy in the temporal pattern of the deglacial warming. Alkenone data suggest that the deglacial warming is punctuated by a decrease in temperature between ∼17 and 15ka BP, corroborating previously published alkenone UK′37 SST records from the southern SCS. Within dating uncertainties, this cooling is coeval with the Heinrich Event 1 (H1) time interval in the North Atlantic region, underscoring the imprints of northern hemisphere forcing on tropical SCS ocean temperatures. The deglacial UK′37 SST minimum is also paralleled by a maximum in G. ruber morphotype-specific δ18O. G. ruber Mg/Ca SST estimates suggest a morphotype-specific record of SSTs during the time interval of H1. Whereas G. ruber s.s. imply a continuous warming starting around 18ka BP without any marked response to H1, G. ruber s.l.-based Mg/Ca SST estimates reveal a cooling around ∼17–15ka BP similar to the H1 interval cooling seen in the alkenone SST record. Similar proxy-dependent differences in deglacial surface water warming have been recorded in the eastern equatorial Pacific, implying a common pattern on both sides of the tropical Pacific Ocean. We submit that this discrepancy could be due to differences in seasonality of planktonic foraminifera G. ruber morphotypes and alkenone-producing algae.

Mg/Ca ratios of two Globigerinoides ruber (white) morphotypes: Implications for reconstructing past tropical/subtropical surface water conditions

Tests of the planktonic foraminifer Globigerinoides ruber (white; d'Orbigny) have become a standard tool for reconstructing past oceanic environments. Paleoceanographers often utilize the Mg/Ca ratios of the foraminiferal tests for reconstructing low‐latitude ocean glacial‐interglacial changes in sea surface temperatures (SST). We report herein a comparison of Mg/Ca measurements on sample pairs (n = 20) of two G. ruber (white) morphotypes (G. ruber sensu stricto (s.s.) and G. ruber sensu lato (s.l.)) from surface and downcore samples of the western Pacific and Indian Oceans. G. ruber s.s. refers to specimens with spherical chambers sitting symmetrically over previous sutures with a wide, high arched aperture, whereas G. ruber s.l. refers to a more compact test with a diminutive final chamber and small aperture. The G. ruber s.s. specimens generally show significantly higher Mg/Ca ratios compared to G. ruber s.l. Our results from the Mg/Ca ratio analysis suggest that G. ruber s.l. specimens precipitated their shells in slightly colder surface waters than G. ruber s.s. specimens. This conclusion is supported by the differences in δ18O and δ13C values between the two morphotypes. Although it is still unclear if these two morphotypes represent phenotypic variants or sibling species, our findings seem to support the hypothesis of depth and/or seasonal allopatry within a single morphospecies.