Frank J C Peeters

Vigorous exchange between the Indian and Atlantic oceans at the end of the past five glacial periods

The magnitude of heat and salt transfer between the Indian and Atlantic oceans through ‘Agulhas leakage’ is considered important for balancing the global thermohaline circulation. Increases or reductions of this leakage lead to strengthening or weakening of the Atlantic meridional overturning and associated variation of North Atlantic Deep Water formation. Here we show that modern Agulhas waters, which migrate into the south Atlantic Ocean in the form of an Agulhas ring, contain a characteristic assemblage of planktic foraminifera. We use this assemblage as a modern analogue to investigate the Agulhas leakage history over the past 550,000 years from a sediment record in the Cape basin. Our reconstruction indicates that Indian–Atlantic water exchange was highly variable: enhanced during present and past interglacials and largely reduced during glacial intervals. Coherent variability of Agulhas leakage with northern summer insolation suggests a teleconnection to the monsoon system. The onset of increased Agulhas leakage during late glacial conditions took place when glacial ice volume was maximal, suggesting a crucial role for Agulhas leakage in glacial terminations, timing of interhemispheric climate change and the resulting resumption of the Atlantic meridional overturning circulation.

On the role of the Agulhas system in ocean circulation and climate

The Atlantic Ocean receives warm, saline water from the Indo-Pacific Ocean through Agulhas leakage around the southern tip of Africa. Recent findings suggest that Agulhas leakage is a crucial component of the climate system and that ongoing increases in leakage under anthropogenic warming could strengthen the Atlantic overturning circulation at a time when warming and accelerated meltwater input in the North Atlantic is predicted to weaken it. Yet in comparison with processes in the North Atlantic, the overall Agulhas system is largely overlooked as a potential climate trigger or feedback mechanism. Detailed modelling experiments—backed by palaeoceanographic and sustained modern observations—are required to establish firmly the role of the Agulhas system in a warming climate.

The effect of upwelling on the distribution and stable isotope composition of Globigerina bulloides and Globigrinoides ruber (planktic foraminifera) in modern surface waters of the NW Arabian Sea.

Abstract Hydrographic changes in the NW Arabian Sea are mainly controlled by the monsoon system. This results in a strong seasonal and vertical gradient in surface water properties, such as temperature, nutrients, carbonate chemistry and the isotopic composition of dissolved inorganic carbon ( δ 13 C DIC ). Living specimens of the planktic foraminifer species Globigerina bulloides and Globigerinoides ruber , were collected using depth stratified plankton tows during the SW monsoon upwelling period in August 1992 and the NE monsoon non-upwelling period in March 1993. We compare their distribution and the stable isotope composition to the seawater properties of the two contrasting monsoon seasons. The oxygen isotope composition of the shells ( δ 18 O shell ) and vertical shell concentration profiles indicate that the depth habitat for both species is shallower during upwelling (SW monsoon period) than during non-upwelling (NE monsoon period). The calcification temperatures suggest that most of the calcite is precipitated at a depth level just below the deep chlorophyll maximum (DCM), however above the main thermocline. Consequently, the average calcification temperature of G. ruber and G. bulloides is lower than the sea surface temperature by 1.7±0.8 and 1.3±0.9 °C, respectively. The carbon isotope composition of the shells ( δ 13 C shell ) of both species differs from the in situ δ 13 C DIC found at the calcification depths of the specimens. The observed offset between the δ 13 C shell and the ambient δ 13 C DIC results from (1) metabolic/ontogenetic effects, (2) the carbonate chemistry of the seawater and, for symbiotic G. ruber , (3) the possible effect of symbionts or symbiont activity. Ontogenetic effects produce size trends in Δ δ 13 C shell–DIC and Δ δ 18 O shell–w : large shells of G. bulloides (250–355μm) are 0.33‰ ( δ 13 C) and 0.23‰ ( δ 18 O) higher compared to smaller ones (150–250 μm). For G. ruber , this is 0.39‰ ( δ 13 C) and 0.17‰ ( δ 18 O). Our field study shows that the δ 13 C shell decreases as a result of lower δ 13 C DIC values in upwelled waters, while the effects of the carbonate system and/or temperature act in an opposite direction and increase the δ 13 C shell as a result lower [CO 3 2− ] (or pH) values and/or lower temperature. The Δ δ 13 C shell–DIC [CO 3 2− ] slopes from our field data are close to those reported literature from laboratory culture experiments. Since seawater carbonate chemistry affects the δ 13 C shell in an opposite sense, and often with a larger magnitude, than the change related to productivity (i.e. δ 13 C DIC ), higher δ 13 C shell values may be expected during periods of upwelling.

Seasonal stratification, shell flux, and oxygen isotope dynamics of left-coiling N. pachyderma and T. quinqueloba in the western subpolar North Atlantic

blooms. This annual population overturning is marked by a 0.7‰ increase in d 18 O in both species. The shell flux of N. pachyderma peaks during the spring bloom and in late summer, when stratification is close to its minimum and maximum, respectively. Both export periods contribute about equally and account for >95% of the total annual flux. Shell fluxes of T. quinqueloba show only a single broad pulse in summer, thus following the seasonal stratification cycle. The d 18 Oo fN. pachyderma reflects temperatures just below the base of the seasonal SML without offset from isotopic equilibrium. The d 18 Op attern ofT. quinqueloba shows a nearly identical amplitude and correlates highly with the d 18 Oo fN. pachyderma. Therefore T. quinqueloba also reflects temperature near the base of the SML but with a positive offset from isotopic equilibrium. These offsets contrast with observations elsewhere and suggest a variable offset from equilibrium calcification for both species. In the Irminger Sea the species consistently show a contrast in their flux timings. Their flux‐weighted Dd 18 O will thus dominantly be determined by seasonal temperature differences at the base of the SML rather than by differences in their depth habitat. Consequently, their sedimentary Dd 18 O may be used to infer the seasonal contrast in temperature at the base of the SML. Citation: Jonkers, L., G.‐J. A. Brummer, F. J. C. Peeters, H. M. van Aken, and M. F. De Jong (2010), Seasonal stratification, shell flux, and oxygen isotope dynamics of left‐coiling N. pachyderma and T. quinqueloba in the western subpolar North Atlantic, Paleoceanography, 25, PA2204, doi:10.1029/2009PA001849.

A size analysis of planktic foraminifera from the Arabian Sea

Abstract Planktic foraminiferal faunas from different environments in the Arabian Sea were size fractionated using 14 sieves with meshes between 100 and 710 μm, to assess the effect of the sieve mesh size cut off level on the faunal composition and to determine the size frequency distribution of individual species. Nine samples from a plankton pump and a towed net, a sediment trap, a box-core and a piston core were selected, to cover living and settling flux faunas as well as fossil faunas from the sediment. In living faunas, most species show an exponential size frequency distribution, with highest numbers in the finest interval of the size spectrum. In sediment trap and core samples, individual species size frequency distributions may consist of: (1) an exponential distribution of relatively small pre-adult specimens; (2) a Gaussian-shaped distribution of larger specimens, which may be classified as adult or terminal; or (3) a combination of both. The distributions are separated using a best fit technique. The composition of the total planktic foraminiferal fauna strongly changes along the size spectrum. Dominant taxa in >355 μm fractions are Orbulina universa, Globorotalia menardii, Globorotalia tumida, Globigerinella siphonifera and Globigerinoides sacculifer, in 125–355 μm fractions Globigerina bulloides, Globigerinoides ruber, Neogloboquadrina dutertrei and Globigerinita glutinata, and in 100 μm) is present in the 100–125 μm fraction and 1–6% is larger than 250 μm. In samples representing a settling flux (sediment trap and sediment samples) 29–57% of the fauna is present in the 100–125 μm fraction, while 6–23% is larger than 250 μm. Size frequency distributions of the dextral Neogloboquadrina complex (= Neogloboquadrina dutertrei and Neogloboquadrina pachyderma + P–D intergrades) show a bimodal pattern; a smaller peak reflecting dextral Neogloboquadrina pachyderma, and a larger peak of adult Neogloboquadrina dutertrei. By applying a best fit technique to the data, the two species may be separated from each other. In size fractions larger than 150 μm most species have reached the adult stage of ontogeny and we recommend this mesh size for standard faunal analysis. In addition, sieve mesh sizes of 125 and 250 μm have to be used to obtain a reliable estimate of the abundance of small and large species, respectively.

Contrasting multiproxy reconstructions of surface ocean hydrography in the Agulhas Corridor and implications for the Agulhas Leakage during the last 345,000 years

Planktonic δ18O and Mg/Ca-derived sea surface temperature (SST) records from the Agulhas Corridor off South Africa display a progressive increase of SST during glacial periods of the last three climatic cycles. The SST increases of up to 4°C coincide with increased abundance of subtropical planktonic foraminiferal marker species which indicates a progressive warming due to an increased influence of subtropical waters at the core sites. Mg/Ca-derived SST maximizes during glacial maxima and glacial Terminations to values about 2.5°C above full-interglacial SST. The paired planktonic δ18O and Mg/Ca-derived SST records yield glacial seawater δ18O anomalies of up to 0.8‰, indicating measurably higher surface salinities during these periods. The SST pattern along our record is markedly different from a U37K′-derived SST record at a nearby core location in the Agulhas Corridor that displays SST maxima only during glacial Terminations. Possible explanations are lateral alkenone advection by the vigorous regional ocean currents or the development of SST contrasts during glacials in association with seasonal changes of Agulhas water transports and lateral shifts of the Agulhas retroflection. The different SST reconstructions derived from U37K′ and Mg/Ca pose a significant challenge to the interpretation of the proxy records and demonstrate that the reconstruction of the Agulhas Current and interocean salt leakage is not as straightforward as previously suggested.

Living Neogloboquadrina pachyderma sin and its distribution in the sediments from Oman and Somalia upwelling areas

Abstract Microfaunal analysis of plankton nets collected offshore Oman/Yemen and sediment traps offshore Somalia shows that Neogloboquadrina pachyderma sin is abundant during the SW monsoon upwelling. The sediment traps recorded the highest flux (136 specimens m−2 day−1) and the highest relative frequency (3.55% of the total foraminifera assemblage) of the species during this period. During the intermonsoon it became less abundant and decreased in size, and only very few N. pachyderma sin were found in the water column during the NE monsoon. Sediment trap and Recent sediment data collected along a downslope transect off Somalia show that the species frequency decreases offshore. The highest concentrations of N. pachyderma sin in plankton nets off Oman/Yemen were found at a depth of 300–500 m. However, the oxygen isotope compositions of N. pachyderma sin at the depth of 300–500 m and from the surface 8 m are identical, and also similar to that of Globigerina bulloides which reflects properties of the upper 25 m of the water column. The carbon isotope values are relatively consistent within the studied samples from both areas except for the specimens from the surface water samples offshore Oman/Yemen. The data show that the δ18O ratio of N. pachyderma sin is in equilibrium with the ambient water while the δ13C values are at least 0.8‰ lower. We infer that N. pachyderma sin in the Arabian Sea reproduces, grows and calcifies in the upper 25 m of the water column, but possibly descends into deeper waters later during its life cycle.

Ocean currents generate large footprints in marine palaeoclimate proxies

Fossils of marine microorganisms such as planktic foraminifera are among the cornerstones of palaeoclimatological studies. It is often assumed that the proxies derived from their shells represent ocean conditions above the location where they were deposited. Planktic foraminifera, however, are carried by ocean currents and, depending on the life traits of the species, potentially incorporate distant ocean conditions. Here we use high-resolution ocean models to assess the footprint of planktic foraminifera and validate our method with proxy analyses from two locations. Results show that foraminifera, and thus recorded palaeoclimatic conditions, may originate from areas up to several thousands of kilometres away, reflecting an ocean state significantly different from the core site. In the eastern equatorial regions and the western boundary current extensions, the offset may reach 1.5 °C for species living for a month and 3.0 °C for longer-living species. Oceanic transport hence appears to be a crucial aspect in the interpretation of proxy signals.

The seasonal and vertical distribution of living planktic foraminifera in the NW Arabian Sea

Abstract The NW Arabian Sea is characterized by a strong seasonal contrast in surface water hydrography. During the SW monsoon of 1992, we encountered strong coastal upwelling characterized by low sea surface temperatures (SST), high nutrient concentrations, a shallow thermocline and a near-surface chlorophyll maximum. By contrast, the hydrography during the NE monsoon of 1993 was characterized by a relatively warm nutrient-depleted surface mixed layer and a deep chlorophyll maximum. We show that the faunal composition, depth habitat and abundance of living planktic foraminifera respond to the hydrographic changes controlled by the seasonally reversing monsoon system. Total shell concentrations (> 125 μm) ranged from 4 to 332 individuals (ind.) m−3 during upwelling and from 3 to 85 ind. m−3 during the non-upwelling season. During upwelling, the fauna was dominated by Globigerina bulloides. During non-upwelling the fauna was characterized by relatively high concentrations of tropical symbiont-bearing species such as Globigerinoides ruber, Globigerinoides sacculifer and Globigerinella siphonifera, whereas concentrations of Globigerina bulloides were an order of magnitude lower. Factor analysis on 15 species yields an upwelling assemblage (UA), a tropical assemblage (TA) and a subsurface assemblage (SA). A fourth factor represents the distribution of the species Globigerina falconensis, which is mainly found in subsurface waters during the non-upwelling period (NE monsoon). A model is presented to calculate the base of the productive zone from the vertical shell concentration profile of a given species. The model is validated by comparing the range in calcification temperatures of G. bulloides, derived from its δ18O, with the in situ sea-water temperature range of the productive zone as predicted from the model. It appears that shell growth (calcite precipitation) is restricted to the productive zone as defined by this method. The average calcification temperature of G. bulloides corresponds to the point of maximum change in the shell concentration profile (i.e. the inflection point). For most shallow-dwelling species, the inflection point is found at or below the depth of the chlorophyll maximum, although above the main thermocline. This study indicates that the depth habitat and abundance of different species varies seasonally. Consequently, the abundance and stable isotope composition of specimens in the fossil record reflects a mixture of specimens that were produced at various depths during the different seasons.

Quantitative estimate of the paleo-Agulhas leakage

The Indian-Atlantic water exchange south of Africa (Agulhas leakage) is a key component of the global ocean circulation. No quantitative estimation of the paleo-Agulhas leakage exists. We quantify the variability in interocean exchange over the past 640,000 years, using planktic foraminiferal assemblage data from two marine sediment records to define an Agulhas leakage efficiency index. We confirm the validity of our new approach with a numerical ocean model that realistically simulates the modern Agulhas leakage changes. Our results suggest that, during the past several glacial-interglacial cycles, the Agulhas leakage varied by ~10 sverdrup and more during major climatic transitions. This lends strong credence to the hypothesis that modifications in the leakage played a key role in changing the overturning circulation to full strength mode. Our results are instrumental for validating and quantifying the contribution of the Indian-Atlantic water leakage to the global climate changes.