Elsa Cortijo

Patterns of Ice‐Rafted Detritus in the Glacial North Atlantic (40–55°N)

The observation by Heinrich (1988) that, during the last glacial period, much of the input of ice-rafted detritus to the North Atlantic sediments may have occurred as a succession of catastrophic events, rekindled interest on the history of the northern ice sheets over the last glacial period. In this paper, we present a rapid method to study the distribution of these events (both in space and time) using whole core low-field magnetic susceptibility. We report on approximately 20 cores covering the last 150 to 250 kyr. Well-defined patterns of ice-rafted detritus appear during periods of large continental ice-sheet extent, although these are not always associated within their maxima. Most of the events may be traced across the North Atlantic Ocean. For the six most recent Heinrich layers (HL), two distinct patterns exist: HL1, HL2, HL4, HL5 are distributed along the northern boundary of the Glacial Polar Front, over most of the North Atlantic between ≈40° and 50°N; HL3 is more restricted to the central and eastern part of the northern Atlantic. The Nd-Sr isotopic composition of the material constituting different Heinrich events indicates the different provenance of the two patterns: HL3 has a typical Scandinavia-Arctic-Icelandic “young crust” signature, and the others have a large component of northern Quebec and northern West Greenland “old crust” material. These isotopic results, obtained on core SU-9008 from the North American basin, are in agreement with the study by Jantschik and Huon (1992), who used K-Ar dating of silt- and clay-size fractions of an eastern basin core (ME-68-89). These data confirm the large spatial scale of these events, and the enormous amount of ice-rafted detritus they represent.

EVIDENCE FOR CHANGES IN THE NORTH ATLANTIC DEEP WATER LINKED TO MELTWATER SURGES DURING THE HEINRICH EVENTS

Abstract Deep sea sediment records from North Atlantic cores (40°N–55°N) provide evidence of several massive iceberg discharges, known as Heinrich events, during the last glacial period. High resolution benthic δ 18 O and δ 13 C records from North Atlantic sediment cores were used to monitor the impact of Heinrich events on thermohaline circulation and to estimate the sensitivity of deep oceanic circulation to changes in freshwater input to the North Atlantic surface waters. Our data indicate that major rearrangements of deep-water masses were directly associated with these massive iceberg discharges. To trace in detail the deep water conditions in the North Atlantic, benthic δ 13 C values in several cores were used to generate time slices before, during and after Heinrich event 4 dated at ∼ 35 ka BP. Although North Atlantic Deep Water continued to form during the oxygen isotope stage 3 at 37 ka BP, deep circulation was characterized by an increased incursion of deep waters of southern origin, which reduced the δ 13 C composition of North Atlantic deep waters, particularly in the eastern Atlantic basin. North Atlantic Deep Water production was reduced during Heinrich Layer 4 (HL4 at ∼ 35 ka BP) synchronously with the changes in the surface water hydrology. Deep convection processes may have occurred in areas not affected by the salinity decrease. Soon after HL4 ( ∼ 33 ka BP) the δ 13 C distribution was similar to that before the event. Similarly, a rapid return to initial δ 13 C values was observed at the end of the most clearly defined Heinrich events (HL5, HL4 and HL1). Comparison between deep circulation patterns corresponding to HL4, the Last Glacial Maximum and Heinrich event 1 indicates that each of these periods was characterized by a different circulation state associated with changes in convection sites.

Millennial‐scale iceberg discharges in the Irminger Basin during the Last Glacial Period: Relationship with the Heinrich events and environmental settings

High-resolution records of coarse lithic content and oxygen isotope have been obtained in a piston core from the Irminger Basin. The last glacial period is characterized by numerous periods of increased iceberg discharges originating partly from Iceland and corresponding to millennial-scale instabilities of the coastal ice sheets and ice shelves in the Nordic area. A comparison with midlatitude sediment cores shows that ice-rafted material corresponding to the Heinrich events was deposited synchronously from 40° to 60°N. There are thus two oscillating systems: every 5–10 kyr massive iceberg armadas are released from large continental ice caps, whereas more frequent instabilities of the coastal ice sheets in the high latitude regions occur every 1.2–3.8 kyr. At the time of the Heinrich events the synchroneity of the response from all the northern hemisphere ice sheets attests the existence of strong interactions between the two systems.

Apparent long-term cooling of the sea surface in the northeast Atlantic and Mediterranean during the Holocene.

Reconstructions of upper ocean temperature (T) during the Holocene (10–0 ka B.P.) were established using the alkenone method from seven, high accumulation sediment cores raised from the northeast Atlantic and the Mediterranean Sea (361N–751N). All these paleo-T records document an apparent long-term cooling during the last 10 kyr. In records with indication of a constant trend, the apparent cooling ranges from � 0.27 to � 0.151C kyr � 1 . Records with indication of time-variable trend show peak-to-peak amplitudes in apparent temperatures of 1.2–2.91C. A principal component analysis shows that there is one factor which accounts for a very large fraction (67%) of the total variance in the biomarker paleo-T records and which dominates these records over other potential secondary influences. Two possible contributions are (1) a widespread surface cooling, which may be associated with the transition fromthe Hypsithermal interval ( B9–5.7 ka B.P.) to the Neoglaciation (B5.7–0 ka B.P.); and (2) a change in the seasonal timing and/or duration of the growth period of alkenone producers (prymnesiophyte algae). The first contribution is consistent with many climate proxy records from the northeast Atlantic area and with climate model simulations including Milankovitch forcing. The second contribution is consistent with the divergence between biomarker and summer faunal paleo-T fromearly to late Holocene observed in two cores. Further work is necessary, and in particular the apparent discordance between biomarker and faunal T records for the relative stable Holocene period must be understood, to better constrain the climatic and ecological contributions to the apparent cooling observed in the former records. r 2002 Elsevier Science Ltd. All rights reserved.

Changes in sea surface hydrology associated with Heinrich event 4 in the North Atlantic Ocean between 40° and 60°N

Abstract The changes in distribution of sea surface temperature and salinity in the North Atlantic between 40 and 60°N were reconstructed for the time interval between 40 and 30 kyr BP, which includes the large iceberg discharge event associated with the deposition of Heinrich layer 4. We found that the meltwater input during deposition of Heinrich layer 4 resulted in a 1–2 kyr temperature decrease of about 2°C and a salinity decrease in the range of 1.5‰–3.5‰ between 40 and 50°N. Sites above 50°N did not experience significant salinity variations. A much larger area was affected by the reduction in sea surface temperature. The amplitude of the sea surface temperature shift was, however, much smaller than the atmospheric temperature changes over Greenland at GISP and GRIP sites.

Variability of the North Atlantic thermohaline circulation during the last interglacial period

Studies of natural climate variability are essential for evaluating its future evolution. Greenland ice cores suggest that the modern warm period (the Holocene) has been relatively stable for the past 9,000 years. Much less is known about other warm interglacial periods, which comprise less than 10% of the climate record during the past 2.5 million years. Here we present high-resolution ocean sediment records of surface and deep-water variables from the Bermuda Rise spanning the last interglacial period, about 118,000–127,000 years ago. In general, deep-water chemical changes are coincident with transitions in surface climate at this site. The records do not show any substantial fluctuations relative to the much higher variability observed during the preceding and subsequent cool climates. The relatively stable interglacial period begins and ends with abrupt changes in deep-water flow. We estimate, using 230Th measurements to constrain the chronology, that transitions occur in less than 400 years.

Zooming in on Heinrich layers

Theories explaining the origin of the abrupt, massive discharges of ice-rafted detritus (IRD) into the glacial North Atlantic (the Heinrich layers (HLs)) generally point to the Laurentide ice sheet as the sole source of these events, until it was found that the IRDs also originated from Icelandic and European ice sheets [Bond and Lotti, 1995; Snoeckx et al., 1999; Grousset et al., 2000]. This apparent contradiction must be reconciled as it raises fundamental questions about the mechanism(s) of HL origin. We have analyzed two ∼12 cm thick HLs in an ultrahigh-resolution mode (1–2 century intervals) in a mid-Atlantic ridge piston core. The δ18O record (N. pachyderma left coiling) reveals strong excursions induced by the melting of the icebergs; these excursions are associated with a strong decrease in the amount of planktic foraminafersand with a 3°C cooling of the surface waters. Counts of coarse detrital grains reveal that IRD are deposited according to a typical sequence (1) volcanic glass, (2) quartz and feldspars, (3) detrital carbonate, that implies a chronology in the melting of the differentpan-Atlantic ice sheets. Sr and Nd isotopic composition confirm that in both Heinrich layers H1 and H2, “precursor” IRD came from first Europe/Iceland, followed then by Laurentide-derived IRD. An internal cyclicity can be identified: during H1 and H2, about four to six major, abrupt discharges occurred roughly on a century timescale. The δ13C and δ15N records reveal that dominant inputs of continent-derived organic matter are associated with IRD within the HLs, hiding the plankton productivity signal.

Changes in Meridional Temperature and Salinity Gradients in the North Atlantic Ocean (30°–72°N) during the Last Interglacial Period

Eight deep-sea sediment cores from the North Atlantic Ocean ranging from 31° to 72°N are studied to reconstruct the meridional gradients in surface hydrographic conditions during the interval of minimum ice volume within the last interglacial period. Using benthic foraminiferal δ18O measurements and estimates of Sea Surface Temperature (SST) and Sea Surface Salinity (SSS), we show that summer SSTs and SSSs decreased gradually during the interval of minimum ice volume at high-latitude sites (52°–72°N) whereas they were stable or increased during the same time period at low-latitude sites (31°–41°N). This increase in meridional gradients of SSTs and SSSs may have been due to changes in the latitudinal distribution of summer and annual-average insolation and associated oceanic and atmospheric feedbacks. These trends documented for the Eemian ice volume minimum period are similar to corresponding changes observed during the Holocene and may have had a similar origin.

Constraints on the duration and freshwater release of Heinrich event 4 through isotope modelling

Heinrich events—abrupt climate cooling events due to ice-sheet instability that occurred during the last glacial period—are recorded in sediment cores throughout the North Atlantic Ocean. Modelling studies have described likely physical mechanisms for these events, but the quantitative characteristics of Heinrich events are less well known. Here we use a climate model of intermediate complexity that explicitly calculates the distribution of oxygen isotopes in the oceans to simulate Heinrich event 4 at about 40,000 yr ago. We compare an ensemble of scenarios for this Heinrich event with oxygen isotope data measured in foraminiferal calcite of a comprehensive set of sediment cores. From this comparison, we obtain a duration of 250 ± 150 yr and an ice release of 2 ± 1 m sea-level equivalent for Heinrich event 4, significantly reducing the uncertainties in both values compared to earlier estimates of up to 2,000 yr and 15 m of sea-level equivalent ice release, respectively. Our results indicate that the consequences of Heinrich events may have been less severe than previously assumed, at least with respect to Greenland climate and sea level.

A biomarker approach to the organic matter deposited in the North Atlantic during the last climatic cycle

Abstract The study of the composition of total organic carbon (TOC), C37 alkenones, and C23C33 n-alkanes in the North Atlantic Ocean (cores SU90/08 and SU90/39) has allowed the development of a model for the differentiation of marine and terrigenous TOC. This model gives rise to results in good agreement with inorganic markers such as magnetic susceptibility (MS) and non-carbonate content. According to this model, the terrigenous TOC accounts for most of the organic matter in the glacial sediments. Thus, the higher TOC of the glacial periods (0.1–0.45% vs. 0.05–0.15% in the interglacials) is due to the increase in terrigenous TOC. The changes in marine TOC (those associated to sea-surface productivity) are independent of the glacial-interglacial evolution. The terrigenous TOC is more important at higher latitudes, probably due to higher terrigenous detrital inputs associated with iceberg transport. In this respect, the correlation between n-alkanes and MS strongly suggests that the main source of these hydrocarbons are ice-rafted materials and that aeolian inputs only represent minor contributions. The four peaks of reworked n-alkanes in the SU90/08 core that are coincident with the Heinrich Layers (H1, H2, H4, and 115) are in agreement with this hypothesis. On the other hand, the abrupt marine TOC peaks of SU90/08 show a 23 ka periodicity, indicating that the marine productivity at 43°N in the North Atlantic Ocean was precessionally driven. The productivity maxima in this core correlate with low latitude oceanographic processes, such as periods of enhanced trade wind intensity and equatorial upwelling which are also tuned to precession and independent of the global glacial-interglacial evolution.