The Marine Record of Continental Tectonics and Erosion Network

Team leaders:

Additional contributors at VU Amsterdam: Prof Dick Kroon, Gerald Ganssen, Dept. Paleoecology and Paleoclimatology

Post docs employed on this grant at Amsterdam; Viviane Bout, Simon Jung and Catherine Pomies

Clay Minerals in Deep Sea Sediments as proxies for oceanographic and climate change.

Isotope studies on ferromanganese crusts clearly reflect oceanographic reorganization and variability of erosional input into the ocean on timescales of 10⁵-10⁶ years, but to resolve detailed changes on shorter timescales requires a different archive. Therefore, Pb- Nd-, and Sr-isotopic compositions of the terrigenous fraction in Late Quaternary sediments have been examined to see if they act as proxies for changes in the provenance of detrital sources to the oceans. The approach focuses on the clay-size fraction (<2 mm).  Two distinct regions have been studied. The North Atlantic, where clays represents a large part of the bulk sediment and the Arabian Sea, where aeolian derived palygorskite is the dominant clay mineral and clays represent <5% of the sediment.

North Atlantic

This fine-grained fraction of N. Atlantic sediments is predominantly delivered from the surrounding continents to the deep-sea by deep-ocean circulation, in contrast to silt- or sand-size particles, which are mainly transported via the atmosphere and the surface circulation of the ocean. This particular aspect of the network project was initiated in the North Atlantic because formation of NADW is one of the fundamental processes that control the thermohaline circulation of the ocean and consequently Earth's climate. Paleoceanographic studies using nutrient proxies have suggested overall changes in oceanographic conditions in the North Atlantic on glacial to interglacial time scales, but reconstructing provenance changes of water masses supplying NADW is difficult using those methods. It has been shown that the mineralogical composition of the clay-size fraction has recorded pronounced glacial-interglacial alternations. Over the last climatic cycle, the most prominent changes in clay mineralogical and marine proxy signatures are associated with the transition from glacial stage 6 to interglacial stage 5 (~130 ka). The beginning of interglacial stage 5, the Eemian, is a close climatic analogue of the Holocene. Sampling, therefore, concentrated on this time period with a wide geographic coverage of the N. Atlantic.

The abundances of the clay minerals clearly follow the Glacial-Interglacial (G-IG) climate change. This variation is accompanied by marked isotopic changes of the clay minerals. Previous studies have suggested that the observed changes in clay mineralogy relate to different physical and chemical weathering conditions during glacial and interglacial times. The new data argue that the coupled changes in clay mineralogy and radiogenic isotope composition of the clay-size fraction reflect changes in source provenances and deep-ocean currents rather than simply weathering conditions. These data therefore establish a major reconfiguration in deep ocean currents at the transition from glacial stage 6 to interglacial stage 5. Changes in clay provenance over time at various sites in the North Atlantic are being determined to map past NADW variability.

Sedimentary Record of Monsoon Variability in NE Africa.

The Asian and African monsoon belong to the most fragile parts of Earth's climate and consequently rapid and large climate changes are predicted that will result in variation in the marine record. The main goal of the project is to reconstruct the W-Asian monsoon history from the sedimentary record based on a multiproxy approach, i.e. to combine classical paleoceanographic measures like stable O- and C-isotopes with proxies for provenance and weathering, i.e., Sr-Nd-Pb-isotopes of the lithic fraction of deep-sea sediments. The latter are used as a dust provenance tool. A combination of this type of seawater- and continental erosion proxy is designed to better understand the coupling of terrestrial and marine climate dynamics.

Sediment core 905 (NW Arabian Sea off Somalia) was chosen for the study due to the high sedimentation rate that reaches up to 36 cm/kyr. Sampling in 0.5 cm steps yields a time resolution of ~ a decade. A detailed stable isotope study of the Holocene of benthic foraminifera was performed as a proxy for the history of intermediate/upper deep water to assess how regional oceanographic/climate changes have influenced the region. These stable isotope data show an overall d¹⁸O-reduction by 0.5 ‰ between 10 and 6.5-7 kyr BP superimposed upon short-term d¹⁸O-variations at a decadal-centennial time scale. The amplitude of these variations is 0.3 ‰ prior, and up to 0.6 ‰ subsequent, to ~ 8.0 kyr BP. Modelling experiments demonstrate that the short-term d¹⁸O-variations between 10 and 6.5-7 kyr BP most likely document changes in the evaporation-precipitation balance in the central Red Sea. Past changes in water temperature and salinity caused the outflowing Red Sea Water to settle up to 800 m deeper than today.

Planktic foraminifera from the Early Holocene record high amplitude and rapid d¹⁸O-change of up to 0.6 ‰ that translates into temperature variations in the Arabian Sea of roughly 2-3^oC during summer. Within the resolution of AMS¹⁴C-dates, these variations occur in phase with precipitation induced d¹⁸O-variations in the Hoti Cave (Oman, Neff et al., 2001). These data imply that solar insolation induced precipitation changes over Oman and controlled temperature variations in the surface waters found off Somalia. These findings imply that the rate and amplitude of natural temperature variations are comparable to, or greater than, those inferred to be a consequence of manmade CO₂ emissions

Based on the stable isotope results specific time intervals were defined to analyse the radiogenic signature of the lithic fraction in detail i.e. the Early Holocene subsection between 10 and 6 kyr BP. Dust transport to the western Arabian Sea almost exclusively occurs during summer. This conclusion is supported by clay mineralogical analysis of samples from core 905 that show a dominance of palygorskite as a typical dust borne clay mineral. Core 905 potentially receives dust from E. Africa (SW-monsoon) and the Arabian peninsula/E-Egypt (Northwesterlies). To deduce the dust input to site 905 we make use of the unique tectono-magmatic histories of different parts of NE-Africa and Arabia.

Constant Nd isotope ratios establish that the dust source for site 905 was constant throughout the Holocene.  Large variations in Sr isotope ratios are interpreted in terms of varying balance between physical and chemical weathering.  The Early Holocene (10-6 kyr) was generally more humid than today (African humid period - AHP) but there were three centennial scale arid-humid oscillations each punctuated by even shorter events. A dramatic change towards dryer conditions between 6 and 3.8 kyr B.P. marks the end of the AHP and dry conditions prevail over the last 3.8 kyrs. Through a comparison with continental and marine records from W. Africa, the AHP is shown to have terminated ~1500 years later in NE- than in NW-Africa. It is proposed that the asynchronous termination of the AHP may result from a “continent-constellation”-effect that is stronger in NE-Africa and facilitates a longer northward shift of the major rainfall belts. The detailed climate record from core 905 provides a reference frame to which human socio-cultural evolution in Africa can now be compared and may help to better understand the development of prehistoric human civilizations in NE Africa.

Neff et al., 2001. Strong coherence between solar variability and the monsoon in Oman 9 and 6 kyr ago. Nature 411, 290-293.

Nd isotopes of foraminifera as a potential paleoceanographic tool?

The use of Nd isotope compositions of planktic foraminifera as a tracer for changes in the composition of sea surface water masses is based on two fundamental assumptions that have yet to be rigorously tested. First that Nd is incorporated during the calcification of the foraminifera test. Second that the Nd isotope composition recorded at the surface is preserved after the forams death and subsequent transport through the water column and storage in the sediment pile. In order to assess the use of this new proxy in paleo-oceanography, we have measured the Nd concentration of foraminifera collected within the water column off-shore Somalia and in the Somali Basin of the Indian Ocean. Multinets and sediment traps were sampled from 20 m to 3045 m depth. Nd partition coefficients (15 to 25) calculated for both species G. ruber and G. bulloides imply that the Nd content of living foraminifera (0.01 ppm) is 100 times lower than that measured in cleaned foraminifera from sediment cores (1 ppm). Changes in the Nd concentration after the forams death potentially reflect Nd addition from deeper in the water column and during diagenesis in sediments. The Nd isotope composition and Nd content of each water masses and sediment are different to the sea surface waters, any Nd addition can disturb the primary Nd isotope composition of foraminifera recorded at the surface. We demonstrate that Nd content of the tests varies after foraminifera death as tests fall down to the sea floor. The efficiency of the cleaning processes currently used to remove secondary coatings from the foraminifera tests seems to be limited by the re-adsorption ability of REE onto calcite. Therefore the isotopic composition measured in cleaned foraminifera from sediment cores represents a mix of the primary isotopic composition recorded at the surface and that of a contamination phase formed in the water column as well as in the sediment. We therefore do not recommend that Nd isotopes be used as paleoceanographic tracers of sea surface water masses.

Publications arising from the Network

Bauch, H.A., Erlenkeuser H., Jung S.J.A  Thiede J (2000): Surface and deep water changes in the subpolar N-Alantic Region during Termination II and the last Interglaciation. Paleoceanography 15 (1), 76-84.

Bout-Roumazeilles V., Debrabant P., Labeyrie L.. Chamley H.  & Cortijo E.. 1997. Latitudinal control of astronomical forcing parameters on the high resolution clay mineral distribution in the 45 to 65^o N range of the North Atlantic Ocean during the past 300,000 years. Paleoceanography, 12, 671-686.

Bout-Roumazeilles V. and Davies G. R.  Clay provenance evidence for a southward shift of North Atlantic deep-water formation in the Labrador Sea during glacial stage 6. In review, Earth Planet. Sci. Lett.

Frank M., Davies G.R., Hofmann A.W. & Claude-Ivanaj C. 2001. Radiogenic isotopes: new tools help reconstruct paleocean circulation and erosion input. EOS 82, 66-71.

Griselin M.,  van Belle C.,  Pomiès C.,  Vroon P.,  van Soest M.C.  and Davies G. R. 2001. Improved chromatographic separation of Nd for application to NdO TIMS measurements, Chemical Geology, 172, p. 347-359.

Jung S. J.A, Davies G. R.  and. Ganssen G.M. Holocene Climate change and instability across N Africa.  In review, Nature.

Jung S.J.A., Ganssen G.M.  and Davies G.R.. Multidecadal variations in the early
Holocene outflow of Red Sea Water into the Arabian Sea. Paleoceanography Vol. 16 , No. 6 , 656-666.

Jung S.J.A., Davies G.R., Ganssen G.M. and Kroon D. Decadal-Centennial scale monsoon variations in the Arabian Sea during the Early Holocene. In review, Science.

Jung S.J.A., Ivanova K., Reichart G.J., Davies G.R., Ganssen G.M., Kroon, D. and van Hinte J.E. Centennial-millenial scale monsoon variations off Somalia over the last 35 kyr. In press, Spec Pub J. Geol Soc London. “ Climate and tectonic evolution of the Arabian Sea”.

Pomiès C, Davies G.R. and Conan S.M-H. Neodymium in Modern foraminifera from the Indian Ocean : Implications for the use of foraminiferal Nd isotope compositions in Paleo-oceanography, Accepted for publication in Earth and Planetary Sci. Lett.

van Krefeld-Sarnthein K., Erlenkeuser M, Grootes, P, Jung S.J.A.,, Nadeau, M.J, Pflaumann, U, Voelker. A.: Potential links between surging ice sheets, circulation changes and the Dansgaard Oeschger cycles in the Irminger Sea, 60-18 kyr.- Paleoceanography (in press)

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Abstracts:

Bout-Roumazeilles V., Davies G.R. and Labeyrie L. 1998. Nd-Sr-Pb evidence of glacial–interglacial variations in clay provenance and transport in the North Atlantic Ocean. Min. Mag. vol. 62a, 1443-4.

Bout-Roumazeilles V., Davies G.R. and Labeyrie L. 1998. Nd-Sr-Pb evidence of stade 6-stade 5 variations in clay provenance and transport in the North Atlantic Ocean. Proc. International Conference on Paleoceanography VI.

Bout-Roumazeilles V., Davies G.R. and Labeyrie L. 1998. Nd-Sr-Pb isotope variations in the main detrital clay input at the 6- 5 transition in the North Atlantic Ocean. EOS Trans., Am. Geophys. Union. 79, p. 512.

Bout-Roumazeilles V., Davies G.R. and Labeyrie L. 1999. Variation in the main detrital clay input at the 6-5 transition in the N. Atlantic ocean revealed by Nd-Sr-Pb isotope study of clay particles. Terra Abs. 11, 174-5.

Bout-Roumazeilles V., Vanderaverot, P, Davies G.R. and Blamart, D. 1999. Does the variation of illite-vermiculite mixed layer minerals input in the N. Atlantic ocean reflect ice-sheet dynamics. Terra Abs. 11, 165-6.

Conan, S.M-H., Brummer G.J.A. and Jung S.J.A. 1998. Fluxes of planktic foraminifera in response to monsoonal upwelling on the Somalia Basin margin, International congress on paleoceanography VI (poster), Lisboa, 1998.

Jung S.J.A., Davies G.R., Ganssen G.M. 2000. Early Holocene sub-centennial monsoon variations in the Arabian Sea; abstract EGS 2000.

Jung, S.J.A., Davies G.R., Ganssen G.M. 2000. Early Holocene sub-centennial mid-depth water variations in the Arabian Sea; abstract EGS 2000.

Jung, S.J.A., Davies G.R., Ganssen G.M. 2001. Early Holocene sub-centennial monsoon variations in the Arabian Sea; (abstract) “Geologic and Climatic Evolution of the Arabian Sea region; Geological Society of London, April 2001.

Jung, S.J.A., Targarona, J., NIOP and Bremen project members 1997. The Arabian Sea and Benguela Upwelling Systems: A synoptic approach. - Poster at NSG annual conference, Amsterdam, December 1997.

Jung S.J.A., Targarona J., NIOP- and Bremen project members. The Arabian Sea and Benguela Upwelling systems: A synoptic approach, International congress on paleoceanography VI (poster), Lisboa, 1998.

Jung, S.J.A., Davies G.R., Ganssen G. M. 2001. Decadal scale climate records: Core 905 off Somalia. – NSG annual conference, Amsterdam 2001.

Pomiès C. and Davies G.R. 2000. Neodymium isotopes in modern foraminifera from Indian Ocean : Assessment of the use of Nd isotope composition of foraminifera as a tracer for paleo-oceanic circulation changes. Rapid Climate Change (Continents/oceans)-Goldschmidt conference-Oxford UK.

Pomiès C., Bout-Roumazeilles V. and Davies G.R. 1999. Glacial-Interglacial record of changing erosion and ocean circulation: Nd isotope study of foraminifera and Clay particles from the North Atlantic. Global ocean deepwater circulation: past, present and future, AGU 2000 Ocean Sciences, San Antonio-Texas-USA. EOS, Transactions, American Geophysical Union, Vol. 80, n° 49, p. OS2.

Pomiès C. 1999 Nd isotope variations of seawater recorded by foraminifera. University of Bern « the dynamics of the Earth System : Processes and records of past climate change ». Abstract Swiss Climate Summer School – Bern-Switzerland.