Massive Iceberg Discharges Research Articles

Role of the thermohaline circulation in the abrupt warming after Heinrich events

EVIDENCE of rapid climate oscillations during the last glacial period has been identified in climate records from Greenland ice cores1,2 and ocean sediments in the North Atlantic3,4. These records show that periods of gradual cooling are terminated by abrupt warming events5, with the coldest periods coinciding with the deposition of ice-rafted debris (so-called Heinrich events) throughout the North Atlantic. Heinrich events are thought to be a signature of massive iceberg discharges owing to collapse of the Laurentide ice sheet; Bond et al.5 have proposed that the decrease in meltwater flux following collapse and retreat of the ice sheet enhanced the ocean's thermohaline circulation6, thereby increasing advection of heat from the tropics and giving rise to abrupt climate warming. Here we test this idea using a simple ocean model coupled to a model of a periodically surging ice sheet. We find that massive discharges of icebergs first stop the thermohaline circulation because of the consequent freshwater influx, cooling the North Atlantic region. This is followed by a rapid restart of the circulation, leading to abrupt warming. Thus our model can reproduce the qualitative features of the climate oscillations seen in the ice-core and ocean records.

Changes in Atmospheric Circulation and Ocean Ice Cover over the North Atlantic During the Last 41,000 Years

High-resolution, continuous multivariate chemical records from a central Greenland ice core provide a sensitive measure of climate change and chemical composition of the atmosphere over the last 41,000 years. These chemical series reveal a record of change in the relative size and intensity of the circulation system that transported air masses to Greenland [defined here as the polar circulation index (PCI)] and in the extent of ocean ice cover. Massive iceberg discharge events previously defined from the marine record are correlated with notable expansions of ocean ice cover and increases in PCI. During stadials without discharge events, ocean ice cover appears to reach some common maximum level. The massive aerosol loadings and dramatic variations in ocean ice cover documented in ice cores should be included in climate modeling.

Continuous early last glacial palaeoenvironmental record from a tasmanian speleothem based on stable isotope and minor element variations

A columnar stalagmite from a limestone cave in the Florentine Valley, Tasmania, has been dated by three 230Th/ 234U age determinations. Continuous profiles of 170 δ 18O and δ 13C values as well as magnesium and strontium concentrations are presented. δ 18O values indicate temperatures lower than today with a gradual temperature decline in the basal part of the profile punctuated by two short cooling events and culminating in a double temperature minimum between 70 and 60 ka. The profile shows marked similarities with the GISP ice core record from Greenland which shows two short intense stadials centred on 74 and 70 ka preceding the Last Glacial temperature minimum. They have been correlated with ‘Heinrich’ events in North Atlantic marine cores which in turn have been attributed to periodic massive discharges of icebergs into the North Atlantic. The Tasmanian speleothem record suggests that these events may also have affected climates in the southern hemisphere. The δ 13C variations also appear to indicate periodic environmental change. Minor element variations were found to be significantly correlated with isotopic changes. Strontium concentrations show a bimodal distribution indicating control by a switching mechanism tentatively attributed to variations in calcitic dust supply to the ground surface above the site.

Evaluating the role of climate cooling in iceberg production and the Heinrich events

BOND et al.1 recently presented evidence for the frequent occurrence, in the Pleistocene epoch, of periods of massive iceberg discharge into the North Atlantic ocean ('Heinrich events'), each lasting a few thousand years. The cause of these events is uncertain, but one possibility1 is repeated advances of the Laurentide ice sheet during episodes of cooler climate. Here I examine this idea, using a model of the Laurentide ice sheet driven by orbitally forced variations in insolation, with and without three additional 3,000-yr-long cooling episodes, imposed just before the occurrence of the three most recent Heinrich events (HI, H2 and H3). In the model, the cooling event preceding HI (when the climate was relatively warm and deglaciation was about to begin) led to increased calving, but those preceding H2 and H3 (which occurred during the last ice age, when the climate was very cold) led to reduced iceberg calving. It thus seems unlikely that the Heinrich events generally reflect a direct response of the Laurentide ice sheet to climate cooling.

Evidence for massive discharges of icebergs into the North Atlantic ocean during the last glacial period

SEDIMENTS in the North Atlantic ocean contain a series of layers that are rich in ice-rafted debris and unusually poor in foraminifera1. Here we present evidence that the most recent six of these 'Heinrich layers', deposited between 14,000 and 70,000 years ago, record marked decreases in sea surface temperature and salinity, decreases in the flux of planktonic foraminifera to the sediments, and short-lived, massive discharges of icebergs originating in eastern Canada. The path of the icebergs, clearly marked by the presence of ice-rafted detrital carbonate, can be traced for more than 3,000 km—a remarkable distance, attesting to extreme cooling of surface waters and enormous amounts of drifting ice. The cause of these extreme events is puzzling. They may reflect repeated rapid advances of the Laurentide ice sheet, perhaps associated with reductions in air temperatures, yet temperature records from Greenland ice cores appear to exhibit only a weak corresponding signal. Moreover, the 5–10,000-yr intervals between the events are inconsistent with Milankovitch orbital periodicities, raising the question of what the ultimate cause of the postulated cooling may have been.