Antoon Kuijpers

Tracking the Atlantic Multidecadal Oscillation through the last 8,000 years.

Understanding the internal ocean variability and its influence on climate is imperative for society. A key aspect concerns the enigmatic Atlantic Multidecadal Oscillation (AMO), a feature defined by a 60- to 90-year variability in North Atlantic sea-surface temperatures. The nature and origin of the AMO is uncertain, and it remains unknown whether it represents a persistent periodic driver in the climate system, or merely a transient feature. Here, we show that distinct, ∼55- to 70-year oscillations characterized the North Atlantic ocean-atmosphere variability over the past 8,000 years. We test and reject the hypothesis that this climate oscillation was directly forced by periodic changes in solar activity. We therefore conjecture that a quasi-persistent ∼55- to 70-year AMO, linked to internal ocean-atmosphere variability, existed during large parts of the Holocene. Our analyses further suggest that the coupling from the AMO to regional climate conditions was modulated by orbitally induced shifts in large-scale ocean-atmosphere circulation.

Millennial-scale glacial variability versus Holocene stability: changes in planktic and benthic foraminifera faunas and ocean circulation in the North Atlantic during the last 60 000 years

Two piston cores, DS97-2P from the Reykjanes Ridge in the central North Atlantic Ocean (1685 m water depth) and ENAM33 from southwest of the Faeroe Islands in the NE Atlantic (1217 m water depth), have been investigated for their planktic and benthic foraminiferal content. DS97-2P is situated near the Subarctic Front and productivity measured by accumulation rates of benthic and planktic foraminifera has been generally high during the Holocene. The productivity shows a clear decrease from an early Holocene maximum to a late Holocene minimum. Coeval changes in the benthic faunas indicate that the food supply changed from large, irregular pulses during the early Holocene to a more sustained flux during the late Holocene. Presumably in concert with decreasing bottom current activity oxygen conditions in the bottom water became poorer. Another feature of the late Holocene is an increasing instability of the North Atlantic thermohaline circulation regime. Nevertheless, the changes in faunal composition and productivity during the Holocene were gradual as compared to the discontinuous distribution patterns and abrupt productivity shifts during the glacial. The glacial shifts were on a millennial time scale and correlate with the interstadial-stadial phases of the Dansgaard-Oeschger cycles in the Greenland ice cores. The faunas of the warm interstadial phases resembled the Holocene faunas, and both surface and bottom productivity was high. The faunas suggest that the interstadial circulation pattern was very similar to the modern system with convection in the Nordic seas and generation of North Atlantic Deep Water. The planktic faunas during the cold stadials and Heinrich events were completely dominated by the polar species Neogloboquadrina pachyderma s, and surface conditions were cold and the productivity low. The benthic faunas were dominated by species that presently thrive in areas with a low amount of food and reduced oxygen content. The water column was probably stratified with low saline, cold surface water overlying poorly aerated, intermediate water masses.

Ocean circulation and iceberg discharge in the glacial North Atlantic: Inferences from unmixing of sediment size distributions

Variability in iceberg discharge and deep-ocean circulation in the North Atlantic during the last glacial period is inferred from the grain-size distribution and trace elemental composition of terrigenous sediments in a deep-sea core taken on Reykjanes Ridge, south of Iceland. End-member modeling of the grain-size distributions is used to unmix the signals of varying bottom-current speed and iceberg discharge. The size distribution within the silt fraction appears to be influenced by both factors. Based on Th-Sc-La relationship, we established that during the ice-rafted detritus events, continental material of likely Greenlandic origin increased to 87%, and that bottom-current–derived material contains to 40% mid-oceanic ridge fines, probably of Icelandic origin. Our results have important implications for the use of silt grain size as an indicator for paleocurrent speed in the glacial North Atlantic. We show that reconstructions of variations in bottom-current speed based on the raw grain-size data are opposite to inferences from the unmixed record. The latter indicates that deep-water convection decreased during periods of enhanced iceberg discharge, which is in general agreement with paleoceanographic reconstructions of the North Atlantic.

The Faroe-Shetland Gateway: Late Quaternary water mass exchange between the Nordic seas and the northeastern Atlantic

Abstract Thirteen piston and gravity cores from the Faroe–Shetland area were investigated for their planktic and benthic foraminiferal and oxygen isotopic distributions. Eight time-slices between 18 ka BP and the present were reconstructed to study variations in surface and deep water exchange between the SE Norwegian Sea and the northeast Atlantic Ocean. Today, a relatively strong northward flow of warm North Atlantic surface water is counterbalanced by a southward outflow of newly convected cold bottom water, the Norwegian Sea Overflow Water. During the last glacial maximum at 18 ka BP both the surface and bottom flows were slow and the climate conditions were Arctic. The convection north of the Faroe area was weak and unstable. The first indication of the deglaciation is a decrease in the planktic oxygen isotope values discernible southwest of the Faroe Islands at 15.5 ka BP. The deglaciation proceeded northeast and eastward synchronous with a gradual intensification of northward flowing warmer Atlantic Intermediate Water along the sea bottom. Meltwater fluxes increased between 14 and 13 ka BP producing cold surface waters, and the climatic cooling was extreme. There was no southward overflow of cold bottom water during this time period and the exchange of water masses between the Nordic seas and the North Atlantic Ocean was essentially reversed, i.e. estuarine. During the Bolling Interstadial at 12.5 ka BP northward flowing warm surface water was present to the east of the Faroe–Shetland Channel, wedged below a tongue of polar water spreading from the northwest and reaching into the Faroe–Shetland Channel. Convection in the Nordic seas and overflow of cold deep water started during the Bolling Interstadial. The polar water spread more eastward and southward during the following cold spell, the Younger Dryas, around 10.3 ka BP. The polar water was overlying the warmer, but more saline Atlantic water, which flowed northward below the cold surface water. The overflow of cold bottom water was supposedly only slightly weaker than during the Bolling Interstadial. Strong inflow of warm surface water took place during the Early Holocene at 9.5 ka BP and relatively dense cold water flowed southward along the bottom. The rate of water mass exchange reached a maximum at 6.5 ka BP, when both the inflow of warm Atlantic surface water and the outflow of cold dense bottom water appear to have been stronger than today.

Geochemical characteristics and provenance of late Quaternary sediments from the Madeira Abyssal Plain, N Atlantic

Summary Late Quaternary sediments of the Madeira Abyssal Plain (MAP) consist of alternations of metre-thick distal turbidites and thin (centimetre to decimetre) pelagic clays, marls and oozes. The geochemical characteristics of 29 turbidites are described using major- and trace-element data from more than 350 samples. These were obtained from two representative piston cores located in the NE and W central parts of the MAP. Three separate groups of turbidites are defined by the geochemical data: (a) organic-rich, (b) ‘volcanic’ and (c) calcareous turbidites. ‘Organic’ turbidites contain 0.3–2% organic carbon. Their sediment originated from the lower continental slope of NW Africa N of 20°N, although one turbidite may have been derived from S of this latitude. ‘Volcanic’ turbidites contain a large proportion of volcaniclastic material and are distinguished by their high TiO2 contents (about 1.5% on a carbonate-free basis). Much of their sediment originated from the oceanic islands of the Canaries and Madeira to the E. Calcareous turbidites are composed predominantly of pelagic carbonate and are defined by high CaCO3 values (more than 75%). Their sediment was derived from the Great Meteor-Cruiser Seamount Chain, W of the MAP.

Late Quaternary sedimentary processes and ocean circulation changes at the Southeast Greenland margin

A study has been made of late Quaternary depositional processes and bottom current activity on the Southeast Greenland margin, using seismic, sub-bottom profiling and deep-tow side-scan sonar data as well as sediment core information. The seabed data demonstrate the occurrence of strong, southerly bottom currents prevailing on the slope and rise. Well-defined longitudinal bedforms indicate maximum mean near-bottom current velocities of up to at least 1.0 m/s at the depth stratum of Labrador Sea water (800–1500 m). Similarly strong currents occur in Denmark Strait overflow water (DSOW) at the base of the slope, whereas more basinward the maximum DSOW flow speed is lower. Iceberg plow marks were found down to about 700 m water depth. Both at the shelf edge and on the lower slope and rise the seafloor morphology is indicative of downslope sediment transport and mass flow deposition, which is concluded to be a typically glacial feature. After generally more sluggish deep-water circulation during the last glacial maximum, DSOW basin ventilation was re-established shortly before 13.3 ka. On the shelf, in front of the retreating Greenland ice margin, permanent or semi-permanent sea ice conditions prevailed until about 12.5 ka. At that time increased Irminger Current activity had resulted in warming, and East Greenland Current (EGC)-controlled iceberg drift increased. No evidence was found for a return to extreme glacial conditions or a ceasing of DSOW flow during the Younger Dryas. Abundant coarse IRD collected at greater water depth from shallow sub-seabed strata has a provenance mainly in the northern part of East Greenland (68–73°N), which demonstrates the existence of a pre-Holocene EGC system initially extending to beyond the shelf edge.

A centennial-scale variability of tropical North Atlantic surface hydrography during the late Holocene

Abstract Sea-surface temperature (SST) and sea-surface salinity (SSS) fluctuations in the northeastern Caribbean have been reconstructed through the last 2000 yr using an artificial neural network and δ 18 O analyses of planktonic foraminifera. A warmer period prevailed in the NE Caribbean from AD ∼700–950, which may reflect the occurrence of stronger and/or more frequent El Nino events. A ∼2°C cooling of winter SSTs, from AD ∼1400 to 1550, coincides with the occurrence of reduced solar output, the Sporer event. Episodes of lower SSSs with marked minima at the onsets of the Dark Ages in Europe (AD ∼500–600) and Little Ice Age (AD ∼1400) are cyclically recurrent at intervals of 200–400 yr, and coincide with drier periods in Mexico. This may indicate that the tropical Atlantic evaporation–precipitation budget and SSSs are affected by a centennial-scale modulation involving the freshwater export (import) from (into) the Atlantic Ocean. Coeval changes recorded in the deep North Atlantic circulation indicate that low-latitude SSS anomalies may be advected polewards by the North Atlantic current system, thus affecting deep-ocean convection and strength of the thermohaline circulation.

A 100 yr record of ocean temperature control on the stability of Jakobshavn Isbrae, West Greenland

An understanding of the interaction between ice sheet dynamics and forcing mechanisms, such as oceanic and atmospheric circulation, is important because of the potential contribution of these processes to constraining models that seek to predict future rates of sea-level change. Here we report new benthic foraminiferal data from Disko Bugt, West Greenland, showing a close correlation between subsurface ocean temperature changes and the ice margin position of the glacier Jakobshavn Isbrae over the past 100 yr. In particular, our faunal data show that warm ocean currents entered a bay, Disko Bugt, during the retreat phases of Jakobshavn Isbrae from A.D. 1920 to 1950 and since 1998. We also show a link between West Greenland ocean temperature and the Atlantic Multidecadal Oscillation, a key climate indicator in the North Atlantic Ocean. The close coupling between the oceans and the cryosphere identified here should be assessed in future projections of sea-level change.

Late warming and early cooling of the sea surface in the Nordic seas during MIS 5e (Eemian Interglacial)

Abstract Geochemical identification of a tephra layer found in two cores from the NE Atlantic Ocean and the SE Norwegian Sea, respectively, and dated to 127 ka BP has enabled us to obtain a precise correlation across the Iceland–Scotland Ridge at the Marine Isotope Stage (MIS) 6/5 transition. The direct distance between the two cores is only about 200 km . South of the Iceland–Scotland Ridge, sea surface temperatures rose abruptly at 130 ka BP at the onset of MIS 5e and at least 2–3000 years earlier than north of the ridge. Maximum sea surface temperatures south of the ridge occurred during this initial phase of MIS 5e, when temperatures in the Nordic Seas were still low. North of the ridge, the sea surface warmed rapidly at 127 ka BP. Correlations between the North Atlantic records and the Eemian of Northwest Europe tentatively indicate that the initial phase of MIS 5e correlates with the early part of the Eemian characterised by a warm, continental type of climate. The period after the warming of the Nordic seas corresponds to the slightly cooler and more oceanic middle Eemian interval in Europe. The sea surface temperatures fell gradually north of the ridge during the later part of MIS 5e and they were low during MIS 5d–5a. South of the ridge the temperatures remained relatively high. The data shows that there was no outflow of deep water from the Norwegian Sea during the later part of MIS 6. Outflow began at the MIS 6/5 transition simultaneous with the sea surface warming south of the ridge.

Sediments and sedimentation at the NE Faeroe continental margin; contourites and large-scale sliding

Abstract High-resolution seismic profiles of the NE Faeroe continental margin show a wedge-shaped sedimentary sequence of up to 2 km thickness on top of lower Tertiary basaltic basement. The sedimentary section can be divided into four major seismic sequences separated by unconformities. The uppermost upper slope deposit is interpreted as a contourite which formed later than the Mid-Miocene by the southerly flow of bottom water along the margin of the SW Norwegian Sea, which is subsequently deflected to the east by the NE Faeroe continental margin and Fugloy Ridge. A scoured channel about 85–90 m deep at about 1000 m water depth associated with the contourite partially follows and modifies a preexisting slump scar. Large-scale slumping and sliding of the middle and lower continental slope below 1500 m water depth have affected sediments of presumably Miocene, Pliocene and Quaternary age. A TOBI deep-tow side-scan sonar mosaic composed of ten lines recorded in 1995 semi-parallel to the slope, shows that mass flow deposits are partially covered by recent contourite sediments on the middle slope. The slump complex at the middle/lower continental slope is younger and has a steep, 250–300 m, irregular, deep main scarp and very large, sometimes rotated blocks near the main scarp. At the base of the slide, numerous 10–15 km long, narrow tracks with individual blocks at the end are found. Several types of debris flows have been mapped, some with a longitudinal flow fabric.