Dick Kroon

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.

A resolution for the coiling direction paradox in Neogloboquadrina pachyderma.

We present new data on genotypic differences and biogeographic distribution of coiling types in the living planktonic foraminiferal morphospecies Neogloboquadrina pachyderma. The genetic evidence demonstrates that coiling direction in N. pachyderma is a genetic trait, heritable through time, and is not a morphological feature reflecting ecophenotypic variation. The two opposite coiling morphotypes appear to have diverged during the late Miocene, and they have distinctly different ecologies. In combination with fossil evidence, biogeography, and ecology the degree of genetic distinction between the two coiling types of N. pachyderma strongly implies that they should be considered different species. We propose the adoption of the widely recognized name N. incompta for the right coiling morphospecies. The genetic evidence also demonstrates a low level (<3%) of aberrant coiling associated with both morphotypes. The abundance of these aberrant specimens has no relationship with the environment. These findings have important consequences for the use of N. pachyderma and N. incompta as paleoceanographic signal carriers in polar and subpolar waters. Copyright 2006 by the American Geophysical Union.

Eastern Mediterranean surface water temperatures and d18O composition during deposition of sapropels in the late Quaternary

0indices) and d 18 O of planktonic foraminifer calcite (d 18 Ofc) across late Pleistocene sapropel intervals show that d 18 Ofc decreased (between 1 and 4.6%) and SST increased (between 0.7� and 6.7� C). Maximal d 18 Oseawater depletion of eastern Mediterranean surface waters at the transition is between 0.5 and 3.0%, and in all but one case exceeded the depletion seen in a western Mediterranean core. The depletion in d 18 Oseawater is most pronounced at sapropel bases, in agreement with an initial sudden input of monsoon-derived freshwater. Most sapropels coincide with warming trends of SST. The density decrease by initial freshwater input and continued warming of the sea surface pooled fresh water in the surface layer and prohibited deep convection down to ageing deep water emplaced during cold and arid glacial conditions. An exception to this pattern is ‘‘glacial’’ sapropel S6; its largest d 18 Oseawater depletion (3%) is almost matched by the depletion in the western Mediterranean Sea, and it is accompanied by surface water cooling following an initially rapid warming phase. A second period of significant isotopic depletion is in isotope stage 6 at the 150 kyr insolation maximum. While not expressed as a sapropel due to cold SST, it is in accord with a strengthened monsoon in the southern catchment. INDEX TERMS: 1055 Geochemistry: Organic geochemistry; 1620 Global Change: Climate dynamics (3309); 4267 Oceanography: General: Paleoceanography; 9604 Information Related to Geologic Time: Cenozoic; KEYWORDS: Mediterranean Sea, sapropels, sea surface temperatures, oxygen isotopes, Quaternary Citation: Emeis, K.-C., et al., Eastern Mediterranean surface water temperatures and d 18 O composition during deposition of sapropels

Northern Indian Ocean upwelling cells and the stable isotope composition of living planktonic foraminifers

Oxygen and carbon isotope ratios were determined of the shells of living planktonic foraminifers, collected along a west-east transect in northern Indian Ocean surface waters. The δ18O values of all species do not correspond closely to the temperature fluctuations caused by the upwelling of colder subsurface waters. This is explained mainly by rapid warming of the episodically advected colder subsurface waters into the euphotic zone. The carbon isotope composition of shells of the non-spinose Neoqloboquadrina dutertrei, Globorotalia menardii and to a lesser extent Globigerinita glutinata clearly shows depletion in 13C within upwelling areas. The δ13C values of the symbiont-bearing, spinose carnivorous species Globigerinoides trilobus, Globigerinoides ruber and Globigerinella siphonifera only exhibit minor variations. Globigerina bulloides shows an enrichment in 13C. We explained the observed δ13C variations assuming that the species represent different phases of the pulsating upwelling system; N. dutertrei and G. menardii representing its initial phase, when nutrient-rich, 13C-depleted water reaches the surface. G. bulloides invades later, after the phyto- and zooplankton blooms are well developed and the uptake of 12C in the organic material has balanced or even exceeded the amount of 12C carried into the surface waters through upwelling. Thus the 13C/12C ratios of the various species depend on their timing of optimum occurrence determined by the intensity, longevity and corresponding biological activities in the upwelling cycle. Unknown vital effects caused by food source and/or metabolic activities may further influence the 13C/12C ratios of the various species.

Distributional pattern of planktonic foraminifers and pteropods in surface waters and top core sediments of the Red Sea, and adjacent areas controlled by the monsoonal regime and other ecological factors

Living planktonic foraminiferal and pteropod distribution patterns in the western Arabian Sea, Gulf of Aden and Red Sea, collected during two summer cruises (1984, 1985), reflect the hydrographical system that is mainly controlled by a combination of monsoonal winds and evaporation rates. Spinose species constitute the majority of the planktonic foraminiferal assemblages in the Red Sea during both monsoonal seasons. The non-spinose species Globorotalia menardii, Neogloboquadrina dutertrei and Pulleniatina obliquiloculata, which are always abundant in the Arabian Sea, are present only during winter inflow. The intensity and duration of these inflowing surface currents control their distribution pattern. Stable oxygen isotope ratios show that G. menardii survives but ceases to grow north of Bab el Mandeb, while N. dutertrei continues to grow. Trends in the foraminiferal distribution in surface waters compare well with those of the sea floor, as far as larger specimens (>250 μm) are concerned, but differ for the small ones. Surface distribution patterns of small-sized specimens and juvenile/neanic stages of large-sized fully grown species do not correspond to those in the core top samples. The distribution pattern of living pteropods in the Red Sea is closely related to distinct water masses and corresponds to the distribution in top core sediments. Pteropods are absent in the sediments of the Gulf of Aden and the western Arabian Sea due to dissolution. Peak abundances of various pteropods and foraminifers indicate the presence of local upwelling processes in the Bab el Mandeb area. Determining these dynamics allows for the reconstruction of ancient oceanic environments and climatic interactions in the area.

The Great Barrier Reef: The Chronological Record from a New Borehole

ABSTRACT A new borehole, 210 mbsf (meters below sea floor) deep, drilled in Ribbon Reef 5 on the Great Barrier Reef off Cooktown, NE Australia, reveals a shallowing-upwards succession, the younger part of which is punctuated by a series of erosion surfaces. Nine depositional units have been defined by lithological changes and are numbered sequentially from the base of the hole upwards. Aminostratigraphy, magnetostratigraphy, radiocarbon dating, uranium series dating, and modeling together with strontium ratios have been applied in an attempt to establish a chronology of accumulation. Carbonate deposition began about 770 ka ago in a relatively deep-water slope environment and is represented by a series of debris flows. Lithoclasts within these rocks, indicate that older limestones already existed in the area. Subsequent accretion involved the downslope accumulation of grainstones and wackestones, sometimes cross-laminated, characterized by intervals with abundant rhodoliths and scattered, probably reworked, corals. Four units at the base of the hole reflect deposition that probably began during isotope stage 16 and continued through stage 15 from about 770 to about 564 ka. Unit 5 probably extended to stage 11 (about 400 ka), and unit 6 to stage 9 ( 330 ka). Typical reefal associations of corals and calcareous algae were established in this area only above depths of about 100 m in the borehole, units 5-4. The succession is apparently unbroken to an erosion surface at 36 mbsf indicating subaerial emergence. The lack of evidence of emergence below this surface reflects progressive accretion or progradation or both. Two younger erosion surfaces define further periods of lowered sea level. Unit 7 is attributed to deposition during isotope stage 7, but erosion during stage 8 resulted in the preservation of only 8 m of unit 7 limestones. Unit 8 is correlated with stage 5 (125 ka), and unit 9 is interpreted as Holocene (post 7,700 ka). The limited thicknesses of units 7, 8, and 9 are considered to reflect erosion. The progressive shallowing brought the depositional surface within the zone exposed during lowstands, and there is no sedimentological evidence that aggradation was restricted by a lack of accommodation.

Middle and Upper Miocene cyclostratigraphy of downhole logs and short- to long-term astronomical cycles in carbonate production of the Great Bahama Bank

Abstract Ocean Drilling Program Site 1006 is located in the Santaren Channel off the Western slope of the Great Bahama Bank. Downhole logs of sediment physical and chemical properties from Hole 1006A contain repetitive alternations (‘cycles’), which, using the nannofossil and planktic foraminifer biostratigraphy as a guide to the sedimentation rate, we show to be paced by the Earth’s climatic precession. The formation microscanner resistivity image log shows a high-resolution record of the sedimentary cycles, and we use it to measure cycle thicknesses. The record of cycle thickness contains long-term cycles that repeat about every 400 kyr, which we correlate to the 400-kyr cycles in orbital eccentricity. This broad-scale tuning results in an age profile for the core that is largely consistent with previously astronomically tuned biostratigraphic dates. It has previously been established that the Bahama Bank produces and exports carbonate preferentially during sea-level highstands. Each highstand produces one sedimentary cycle, hence we infer that sea level also fluctuates with a mainly precessional periodicity (although obliquity is also found in the log records). We attribute the 400-kyr period in cycle thickness to eccentricity modulation of the amplitude of sea-level fluctuations: a high-amplitude sea-level rise will cause more carbonate to be generated and deposited than a low-amplitude sea-level rise.

Notes on the Foraminiferal Genera Laterostomella De Klasz and Rérat and Streptochilus Brönnimann and Resig

the biserial foraminiferal genera Laterostomella de Klasz & Rérat (1962) and Streptochilus Brönnimann & Resig (1971) have been described from the Miocene of Gabon and from Miocene to Recent levels of the Pacific and Indian Oceans, respectively. Both genera have since been found in the Upper Cenozoic of other regions: Laterostomella at Rockall Bank in the North Atlantic and in Papua-New-Guinea, and modern Streptochilus specimens were collected with plankton nets in the northern part of the Indian Ocean. The apertural characteristics of Laterostomella and Streptochilus show marked similarities as does the general shape of Laterostomella guembeliniformis with some Streptochilus species. However, other Laterostomella species have a very different form and isotopic data indicate that Laterostomella has a benthic and Streptochilus a planktic life habitat. We conclude that both genera are valid. For the first time SEM pictures of Laterostomella species are presented to show morphological variability, surface texture and aperture types.

Geochemistry and morphometry on planktonic foraminifera

About one third of the anthropogenic carbon dioxide (CO2) released into the atmosphere in the past two centuries has been taken up by the ocean. As CO2 invades the surface ocean, carbonate ion concentrations and pH are lowered. Laboratory studies indicate that this reduces the calcification rates of marine calcifying organisms, including planktic foraminifera. Such a reduction in calcification resulting from anthropogenic CO2 emissions has not been observed, or quantified in the field yet. Here we present the findings of a study in the Western Arabian Sea that uses shells of the surface water dwelling planktic foraminifer Globigerinoides ruber in order to test the hypothesis that anthropogenically induced acidification has reduced shell calcification of this species. We found that light, thin-walled shells from the surface sediment are younger (based on 14C and d13C measurements) than the heavier, thicker-walled shells. Shells in the upper, bioturbated, sediment layer were significantly lighter compared to shells found below this layer. These observations are consistent with a scenario where anthropogenically induced ocean acidification reduced the rate at which foraminifera calcify, resulting in lighter shells. On the other hand, we show that seasonal upwelling in the area also influences their calcification and the stable isotope (d13C and d18O) signatures recorded by the foraminifera shells. Plankton tow and sediment trap data show that lighter shells were produced during upwelling and heavier ones during non-upwelling periods. Seasonality alone, however, cannot explain the 14C results, or the increase in shell weight below the bioturbated sediment layer. We therefore must conclude that probably both the processes of acidification and seasonal upwelling are responsible for the presence of light shells in the top of the sediment and the age difference between thick and thin specimens.