Showing papers by "Gerald H. Haug published in 2008"

An abrupt wind shift in western Europe at the onset of the Younger Dryas cold period

Abstract: The Younger Dryas cold reversal during the last glacial termination is one of the most abrupt climate change events observed in the Northern Hemisphere. Analyses of varved lake sediments from western Germany suggest that storminess in the North Atlantic region increased within a single year at this time, providing a mechanistic link between a decrease in the meridional overturning circulation and western European cooling. The Younger Dryas cooling 12,700 years ago is one of the most abrupt climate changes observed in Northern Hemisphere palaeoclimate records1,2,3,4. Annually laminated lake sediments are ideally suited to record the dynamics of such abrupt changes, as the seasonal deposition responds immediately to climate, and the varve counts provide an accurate estimate of the timing of the change. Here, we present sub-annual records of varve microfacies and geochemistry from Lake Meerfelder Maar in western Germany, providing one of the best dated records of this climate transition5. Our data indicate an abrupt increase in storminess during the autumn to spring seasons, occurring from one year to the next at 12,679 yr BP, broadly coincident with other changes in this region. We suggest that this shift in wind strength represents an abrupt change in the North Atlantic westerlies towards a stronger and more zonal jet. Changes in meridional overturning circulation alone cannot fully explain the changes in European climate6,7; we suggest the observed wind shift provides the mechanism for the strong temporal link between North Atlantic Ocean overturning circulation and European climate during deglaciation.

Volcanoes and ENSO over the Past Millennium

Abstract: The controversial claim that El Nino events might be partially caused by radiative forcing due to volcanic aerosols is reassessed. Building on the work of Mann et al., estimates of volcanic forcing over the past millennium and a climate model of intermediate complexity are used to draw a diagram of El Nino likelihood as a function of the intensity of volcanic forcing. It is shown that in the context of this model, only eruptions larger than that of Mt. Pinatubo (1991, peak dimming of about 3.7 W m−2) can shift the likelihood and amplitude of an El Nino event above the level of the model’s internal variability. Explosive volcanism cannot be said to trigger El Nino events per se, but it is found to raise their likelihood by 50% on average, also favoring higher amplitudes. This reconciles, on one hand, the demonstration by Adams et al. of a statistical relationship between explosive volcanism and El Nino and, on the other hand, the ability to predict El Nino events of the last 148 yr without knowled...

Estimated Reservoir Ages of the Black Sea Since the Last Glacial

Abstract: Accelerator mass spectrometry (AMS) radiocarbon dating of ostracod and gastropod shells from the southwestern Black Sea cores combined with tephrochronology provides the basis for studying reservoir age changes in the late-glacial Black Sea. The comparison of our data with records from the northwestern Black Sea shows that an apparent reservoir age of ∼1450 14C yr found in the glacial is characteristic of a homogenized water column. This apparent reservoir age is most likely due to the hardwater effect. Though data indicate that a reservoir age of ∼1450 14C yr may have persisted until the Bolling-Allerod warm period, a comparison with the GISP2 ice-core record suggests a gradual reduction of the reservoir age to ∼1000 14C yr, which might have been caused by dilution effects of inflowing meltwater. During the Bolling-Allerod warm period, soil development and increased vegetation cover in the catchment area of the Black Sea could have hampered erosion of carbonate bedrock, and hence diminished contamination by “old” carbon brought to the Black Sea basin by rivers. A further reduction of the reservoir age most probably occurred contemporary to the precipitation of inorganic carbonates triggered by increased phytoplankton activity, and was confined to the upper water column. Intensified deep water formation subsequently enhanced the mixing/convection and renewal of intermediate water. During the Younger Dryas, the age of the upper water column was close to 0 yr, while the intermediate water was ∼900 14C yr older. The first inflow of saline Mediterranean water, at ∼8300 14C yr BP, shifted the surface water age towards the recent value of ∼400 14C yr.

Expansion of subarctic water masses in the North Atlantic and Pacific oceans and implications for mid-Pleistocene ice sheet growth

Abstract: [1] Past surface ocean circulation changes associated with the mid-Pleistocene transition, 0.9–0.6 Ma, were reconstructed in the northern North Atlantic (ODP 983) and the northwest Pacific (ODP 882), using proxies for subarctic/subpolar water mass distributions (%C37:4 alkenone) and sea surface temperature (U37 ). Both sites experienced a secular expansion of subarctic waters from � 1.15 Ma, spanning both glacial and interglacial intervals. After 0.9 Ma, low %C37:4 at Site 983 records a northward retreat of subarctic waters during interglacials in the Atlantic, while continued high glacial %C37:4 indicate extensive subarctic waters during glacial maxima associated with the development of the larger late Pleistocene ice sheets. In contrast, a secular decline in %C37:4 occurred at Site 882 from 0.9 to 0.5 Ma, marking a more gradual retreat of subarctic conditions in the Pacific. It is proposed that the expansion of subarctic waters between 1.15 and 0.9 Ma exerted negative feedbacks to the moisture supply to the ice sheet source regions and may account for the apparent delayed ice sheet response to atmosphere-ocean circulation changes associated with the mid-Pleistocene transition that began as early as 1.2 Ma.