Planktonic Foraminiferal Oxygen Research Articles

High resolution tropical climate record for the last 6,000 years

Planktonic foraminiferal oxygen isotope records from the Cariaco Basin, offshore Venezuela were used to construct high‐resolution tropical Atlantic climate records for the mid to late Holocene. Our results indicate that major increases in δ18O due both to increases in salinity and decreases in sea surface temperature occurred at least six times over the last 6,000 years (∼6,000–5,000, 4,500–4,200, 3,800–3,200, 3,000–2,800, 2,200–2,000, and 1200–800 cal yrs. B.P.). The initial δ18O increase centered at 5,500 years ago is coincident with the development of arid conditions in the Caribbean region and the end of the “African humid period” indicating a global drying of the northern tropics at this time. Synchronous with the aridification of the Caribbean region are the onset of wetter conditions in the South American Altiplano and the reoccupation of this region by humans. This combination of contrasting climate conditions is attributed to a southward displacement of the Inter‐Tropical Convergence Zone (ITCZ) that would have resulted in decreased precipitation and increased trade wind intensity in the Caribbean region and increased rainfall over the Altiplano.

East Asian monsoon forcing of suborbital variability in the Sulu Sea during Marine Isotope Stage 3: Link to Northern Hemisphere climate

We have generated a new high‐resolution record of variations in planktonic foraminiferal oxygen isotopes (δ18O) and Mg/Ca from a sediment core (IMAGES 97‐2141) in the Sulu Sea located in the Philippine archipelago of western tropical Pacific. This record reveals distinct, suborbital‐scale δ18O changes, most notably during Marine Isotope Stage 3 (MIS3) (∼30,000 to 60,000 years B.P.). The amplitudes of these δ18O fluctuations (0.4 to 0.7‰) exceed that which can be attributed to sea level changes and must be due to changes in sea surface conditions. In the same interval, variations in planktonic foraminifera Mg/Ca suggest that suborbital surface ocean temperature variations of 1 to 1.5°C in the Sulu Sea were not in phase with δ18O. Combined, this evidence indicates that the MIS3 millennial δ18O events in the Sulu Sea were primarily the result of changes in surface water salinity, which today is directly related to the East Asian Monsoon (EAM) and its influence on the balance between surface water contributions from the South China Sea and Western Pacific Warm Pool (WPWP). Within dating uncertainties the MIS3 Sulu Sea δ18O suborbital variability indicates that times of fresher surface conditions in the Sulu Sea coincide with similar conditions in the WPWP [Stott et al., 2002] and also with intensifications of the summer EAM as recorded in the U‐Th dated Chinese (Hulu Cave) speleothem δ18O record [Wang et al., 2001] and thus by inference with interstadials in the Greenland Ice core records. Combined, these results indicate that pronounced suborbital variability in the summer EAM and Intertropical Convergence Zone (ITCZ) during MIS3 was tightly coupled with climate conditions in the northern high latitudes.

Tasman Front shifts and associated paleoceanographic changes during the last 250,000 years: foraminiferal evidence from the Lord Howe Rise

The paleoceanography of the Tasman Sea over the past 250,000 years was studied using benthic (>75 μm size fraction) and planktonic foraminifera (>149 μm size fraction) from three cores collected along ∼162°E traverse between ∼25°S and ∼30°S on the Lord Howe Rise. Planktonic foraminiferal oxygen isotope stratigraphy dates the cores between OIS 1 and 11. R-mode cluster and Q-mode factor analyses were carried out on benthic foraminiferal faunas, and Q-mode factor analysis and the modern analog technique (MAT) were used in analyzing planktonic foraminiferal faunas. Distinct benthic faunas across latitude from north (∼25°S) to south (∼30°S and ∼35°S) reflects the difference in primary productivity level in the overlying surface water. The MAT result is thought to express latitudinal shifts of the Tasman Front over the last 250,000 years with: (1) the Tasman Front at ∼35°S during the oxygen isotope stage (OIS) 1 (post-glacial period); (2) migration of the front nearby ∼25°S during the last glacial period (OIS 2–OIS 4) and slightly northward of its present position during the penultimate glacial period (OIS 6); and (3) a return of the front to near ∼35°S during OIS 5 and OIS 7. Based on time-series and spatial variations of benthic foraminiferal factor typified by Pseudoparrella exigua and Uvigerina peregrina and one variety, southern-winter mixing and convection along the Tasman Front may have strengthened during the interglacial OIS 7 in particular.

Surface ocean density gradients during the Last Glacial Maximum

We have compiled published planktonic foraminiferal oxygen isotope values from the tropical to subtropical Atlantic and Pacific for the late Holocene and Last Glacial Maximum (LGM). We interpret the spatial distribution of δ18O values in terms of meridional density gradients in the LGM surface ocean and discuss implications on tropical ocean thermocline structure. In both oceans, glacial δ18O values display a pattern of interhemispheric asymmetry evidenced by increasingly higher glacial δ18O values toward northern latitudes. The δ18O‐derived density estimates suggest that the northern subtropical surface ocean density was higher during the LGM than today, which has significant implications on LGM thermocline ventilation. In the Atlantic, steeper density gradients in the northern subtropics during the LGM are consistent with a more compressed subtropical gyre and thermocline water formation farther south than today. In the Pacific the steeper density gradients suggest a significant reduction in freshwater fluxes to the northern tropical and subtropical regions, which raises the possibility of a more significant component of Northern Hemisphere source waters to the tropical Pacific thermocline. These predictions could be tested with numerical ocean models that assess the structure of the tropical thermocline in the glacial ocean.

Link between oceanic heat transport, thermohaline circulation, and the Intertropical Convergence Zone in the early Pliocene Atlantic

Planktonic foraminiferal oxygen isotope records from the western and eastern tropical Pacific and Atlantic Oceans suggest a southward shift in the Intertropical Convergence Zone toward its modern location between 4.4 and 4.3 Ma. A concomitant shift in the carbon isotope compositions of Atlantic benthic foraminifera provides strong evidence for an increased thermohaline overturn at this time. We suggest that the southward shift of the Intertropical Convergence Zone and associated change in trade-wind circulation altered equatorial surface hydrography, increased the advection of warmer and more saline surface waters into the subtropical and North Atlantic, and contributed to thermohaline overturn.

Temperature and salinity records of the Tyrrhenian Sea during the last 18,000 years

Abstract We have used the modern analogue technique to derive sea surface temperature (SST) records of the last 18,000 years from two well-dated deep-sea cores recovered in the Tyrrhenian Sea. Results show two major cooling events synchronous with the European and North Atlantic Oldest and Younger Dryas events. A later cooling of lower amplitude is observed between about 8000 and 4500 yr BP. Tyrrhenian SSTs were similar to those of the upper Holocene only during the Bolling/Allerod event and at the beginning of the Holocene. We compared the SST and the planktonic foraminiferal oxygen isotope records to estimate the surface salinity variations. Our reconstruction shows that salinity decreased in the Tyrrhenian Sea during the three cold events, whereas high SSTs correlate with high salinities. During most of the last 18,000 years. Tyrrhenian Sea salinity variations were similar to those of the North Atlantic Ocean near the entrance of the Mediterranean Sea. The salinity gradient between the two basins was thus constant, indicating that the western Mediterranean circulation pattern and the freshwater budget were not significantly different from the modern one. The case was however different during the middle Holocene, when the strong salinity decrease associated with the sapropel deposit in the eastern Mediterranean basin had no equivalent in the North Atlantic Ocean. This peculiar event can only be explained by a local climatic change and an increase of precipitation.

Movement of hydrological fronts and widespread erosional events in the southwestern Tasman sea during the Late Quaternary*

The planktonic foraminiferal palaeotemperatures and oxygen isotopes, radiocarbon dates and calcareous nanno‐plankton biostratigraphy of six deep‐sea cores in the southwestern Tasman Sea are presented, along with the nannoplankton biostratigraphy of the tops of 23 other cores in the region. A complex history of deposition and erosion is indicated, with development of hiatuses at 10 ka, and widespread absence of sediments younger than 73 ka. The history of movement of the major hydrological fronts over the areas west to southwest of Tasmania has been deduced from palaeotemperature records of the undisturbed core segments. During the Last Glacial Maximum, the Subtropical Convergence was positioned north of 42°S, 145°E. During the subsequent deglaciation and Holocene the front rapidly moved southward, with its southern limits established south of 47°S. Presently the northern limit of the Polar Front south of Tasmania is positioned south of ∼50cS, and palaeotemperatures and the absence of silica‐rich sediments in all cores north of there, suggests that the front has been south of there during the past 500 000 years.

A refined Pliocene to early Pleistocene chronostratigraphic frame at ODP Hole 653A (West Mediterranean)

Two planktonic foraminiferal oxygen isotope records of ODP Hole 653A (Tyrrhenian Sea) are presented for the time period extending from approximately 0.8 to 3.0 Ma. Six, generally accepted, synchronous bioevents were used to precise the oxygen isotope chronology and to identify the oxygen isotope stages 22 down to 114. Subsequently, this oxygen isotope chronology was used to determine the synchronism or diachronism of various other biostratigraphic events with those recorded in the Singa and Ficarazzi land sections (Italy) and those in other DSDP/ODP sites.New results concern the diachronity of the FOD of the planktonic foraminiferal species N. atlantica, G. truncatulinoides truncatulinoides and G. inflata between ODP Hole 653A and the Italian landsections. Because many species entered the Mediterranean in short term fluxes, strongly related to the southward migration of cool North Atlantic surface waters, their time distribution through the Pliocene-Pleistocene generally corresponds to alternated intervals of presence and absence. This should explain most of the apparently diachronous appearances and disappearances.Alternating presence-absence patterns are of less importance for the various nannofossil events. The LOD of D. surculus occurs during the transition of stage 100 to 101 in both ODP Hole 653A and the Singa section, which is in perfect agreement with the disappearance of this species from the open ocean. The LOD of D. pentaradiatus in the Mediterranean occurs in stages 100-99, which seems to be consistent with the extinction of this species in the southern Hemisphere. G. oceanica, which corresponds to the 4 μm <Gephyrocapsa spp < 5.5 μm is recorded in stages 65 to 64 at ODP Hole 653A. The Gephyrocapsa spp. > 5.5 μm first occurred in stage 51 at Hole 653A, which fits within the uncertainty interval for this event stretching from stage 51 to 47 in the open ocean and seems therefore a useful tool for conventional biostratigraphy in the Mediterranean.

Tertiary oxygen isotope synthesis, sea level history, and continental margin erosion

Tertiary benthic and planktonic foraminiferal oxygen isotope records are correlated to a standard geomagnetic polarity time scale, making use of improved chronostratigraphic control and additional Oligocene isotope data. Synchronous changes in both benthic and planktonic δ18O values which occurred in the Oligocene to Miocene (36–5.2 Ma) are interpreted, in part, to represent ice growth and decay. The inferred ice growth events correlate with erosion on passive continental margins as interpreted from seismic and chronostratigraphic records. This association is consistent with a link between Oligocene to Miocene erosional events and rapid (&gt;15 m/m.y.) glacioeustatic lowerings of about 50 m. High benthic foraminiferal δ18O values suggest the presence of continental ice sheets during much of the Oligocene to Recent (36–0 Ma). Substantially ice‐free conditions probably existed throughout the Paleocene and Eocene (66–36 Ma). The mechanisms and rates of sea level change apparently were different between the early and late Tertiary, with glacioeustatic changes restricted to the past 36 m.y. Pre‐Oligocene erosion on passive continental margins was caused by eustatic lowerings resulting from global spreading rate changes. We apply a model which suggests that large areas of the continental shelves were subaerially exposed during such tectonoeustatic lowstands, stimulating slope failure and submarine erosion. The different mechanisms and rates of eustatic change may have caused contrasting erosional patterns between the early and late Tertiary on passive continental margins. This speculation needs to be confirmed by examination of data from several passive margins.

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 foraminifera Neogloboquadrina dutertrei and relatively light planktonic foraminiferal δ 18O 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.

Isotopic Paleoceanography of the Caribbean and East Pacific: Role of Panama Uplift in Late Neogene Time

Comparisons of carbon isotopic data on benthic foraminifera from Deep Sea Drilling Project sites 502 (western Caribbean) and 503 (eastern Pacific) indicate that the difference between the Atlantic and the Pacific in the per mil enrichment in carbon-13 of total dissolved carbon dioxide increased about 6 million years ago and again 3 million years ago, when the difference reached the modern level (1 per mil). Comparisons of planktonic foraminiferal oxygen isotopic data for the Caribbean and the Pacific suggest that the salinity of Caribbean surface waters began increasing 4 million years ago, possibly in response to shoaling of the Panama isthmus. These results suggest that modern circulation patterns in the Caribbean and eastern Pacific developed by 3 million years ago in concert with changing tectonic, climatic, and biogeographic patterns.