Rapid changes in the mechanism of ocean convection during the last glacial period

High-amplitude, rapid climate fluctuations are common features of glacial times. The prominent changes in air temperature recorded in the Greenland ice cores are coherent with shifts in the magnitude of the northward heat flux carried by the North Atlantic surface ocean; changes in the ocean's thermohaline circulation are a key component in many explanations of this climate flickering. Here we use stable-isotope and other sedimentological data to reveal specific oceanic reorganizations during these rapid climate-change events. Deep water was generated more or less continuously in the Nordic Seas during the latter part of the last glacial period (60 to 10 thousand years ago), but by two different mechanisms. The deep-water formation occurred by convection in the open ocean during warmer periods (interstadials). But during colder phases (stadials), a freshening of the surface ocean reduced or stopped open-ocean convection, and deep-water formation was instead driven by brine-release during sea-ice freezing. These shifting magnitudes and modes nested within the overall continuity of deep-water formation were probably important for the structuring and rapidity of the prevailing climate changes.

[1]  R. Fairbanks.  A 17,000-year glacio-eustatic sea level record: influence of glacial melting rates on the Younger Dryas event and deep-ocean circulation , 1989, Nature.

[2]  S. Manabe,et al.  Simulation of abrupt climate change induced by freshwater input to the North Atlantic Ocean , 1995, Nature.

[3]  P. Mayewski,et al.  Climate correlations between Greenland and Antarctica during the past 100,000 years , 1994, Nature.

[4]  S. Manabe,et al.  Coupled ocean‐atmosphere model response to freshwater input: Comparison to Younger Dryas Event , 1997 .

[5]  J. Duplessy,et al.  Changes in sea surface hydrology associated with Heinrich event 4 in the North Atlantic Ocean between 40° and 60°N , 1997 .

[6]  W. R. Peltier,et al.  Ice in the Climate System , 1993 .

[7]  R. Henrich.  The younger dryas , 1998 .

[8]  W. Broecker,et al.  Correlations between climate records from North Atlantic sediments and Greenland ice , 1993, Nature.

[9]  G. Bond,et al.  Iceberg Discharges into the North Atlantic on Millennial Time Scales During the Last Glaciation , 1995, Science.

[10]  P. Grootes,et al.  Thermohaline instability in the North Atlantic during meltwater events: Stable isotope and ice-rafted detritus records from Core SO75-26KL, Portuguese Margin , 1997 .

[11]  E. Jansen,et al.  Paleoenvironmental changes in the Norwegian Sea and the northeast Atlantic during the last 2.8 m.y.: Deep Sea Drilling Project/Ocean Drilling Program Sites 610, 642, 643 and 644 , 1988 .

[12]  Scott J. Lehman,et al.  Suborbital timescale variability of North Atlantic Deep Water during the past 200 , 1995 .

[13]  M. Stuiver,et al.  Oxygen 18/16 variability in Greenland snow and ice with 10 -3- to 105-year time resolution , 1997 .

[14]  J. Jouzel,et al.  Evidence for general instability of past climate from a 250-kyr ice-core record , 1993, Nature.

[15]  Wallace Broeker,et al.  The Great Ocean Conveyor , 1991 .

[16]  J. D. Hays,et al.  Age Dating and the Orbital Theory of the Ice Ages: Development of a High-Resolution 0 to 300,000-Year Chronostratigraphy , 1987, Quaternary Research.

[17]  N. Shackleton.  Attainment of isotopic equilibrium between ocean water and the benthonic foraminifera genus Uvigerina: isotopic changes in the ocean during the last glacial , 1974 .

[18]  J. Duplessy,et al.  Variations in mode of formation and temperature of oceanic deep waters over the past 125,000 years , 1987, Nature.

[19]  S. Lehman,et al.  Mid-Depth Circulation of the Subpolar North Atlantic During the Last Glacial Maximum , 1993, Science.

[20]  T. L. Rasmussen,et al.  Circulation changes in the Faeroe-Shetland Channel correlating with cold events during the last glacial period (58–10 ka) , 1996 .

[21]  N. Shackleton.  Oxygen isotopes, ice volume and sea level , 1987 .

[22]  W. Broecker,et al.  Thermohaline circulation, the achilles heel of our climate system: will man-made CO2 upset the current balance? , 1997, Science.

[23]  J. Jouzel,et al.  Comparison of oxygen isotope records from the GISP2 and GRIP Greenland ice cores , 1993, Nature.

[24]  L. D. Meeker,et al.  Major features and forcing of high‐latitude northern hemisphere atmospheric circulation using a 110,000‐year‐long glaciochemical series , 1997 .

[25]  T. Stocker,et al.  Transport of Freshwater into the Deep Ocean by the Conveyor , 1993 .

[26]  M. Bender,et al.  Carbon and oxygen isotopic disequilibria of recent deep-sea benthic foraminifera☆ , 1981 .