Showing papers by "Gerald H. Haug published in 2010"

The polar ocean and glacial cycles in atmospheric CO 2 concentration

Abstract: Global climate and the atmospheric partial pressure of carbon dioxide () are correlated over recent glacial cycles, with lower during ice ages, but the causes of the changes are unknown. The modern Southern Ocean releases deeply sequestered CO(2) to the atmosphere. Growing evidence suggests that the Southern Ocean CO(2) 'leak' was stemmed during ice ages, increasing ocean CO(2) storage. Such a change would also have made the global ocean more alkaline, driving additional ocean CO(2) uptake. This explanation for lower ice-age , if correct, has much to teach us about the controls on current ocean processes.

Subpolar Link to the Emergence of the Modern Equatorial Pacific Cold Tongue

Abstract: The cold upwelling "tongue" of the eastern equatorial Pacific is a central energetic feature of the ocean, dominating both the mean state and temporal variability of climate in the tropics and beyond. Recent evidence for the development of the modern cold tongue during the Pliocene-Pleistocene transition has been explained as the result of extratropical cooling that drove a shoaling of the thermocline. We have found that the sub-Antarctic and sub-Arctic regions underwent substantial cooling nearly synchronous to the cold tongue development, thereby providing support for this hypothesis. In addition, we show that sub-Antarctic climate changed in its response to Earth's orbital variations, from a subtropical to a subpolar pattern, as expected if cooling shrank the warm-water sphere of the ocean and thus contracted the subtropical gyres.

Carbon dioxide effects of Antarctic stratification, North Atlantic Intermediate Water formation, and subantarctic nutrient drawdown during the last ice age: Diagnosis and synthesis in a geochemical box model

Abstract: In a box model synthesis of Southern Ocean and North Atlantic mechanisms for lowering CO2 during ice ages, the CO2 changes are parsed into their component geochemical causes, including the soft-tissue pump, the carbonate pump, and whole ocean alkalinity. When the mechanisms are applied together, their interactions greatly modify the net CO2 change. Combining the Antarctic mechanisms (stratification, nutrient drawdown, and sea ice cover) within bounds set by observations decreases CO2 by no more than 36 ppm, a drawdown that could be caused by any one of these mechanisms in isolation. However, these Antarctic changes reverse the CO2 effect of the observed ice age shoaling of North Atlantic overturning: in isolation, the shoaling raises CO2 by 16 ppm, but alongside the Antarctic changes, it lowers CO2 by an additional 13 ppm, a 29 ppm synergy. The total CO2 decrease does not reach 80 ppm, partly because Antarctic stratification, Antarctic sea ice cover, and the shoaling of North Atlantic overturning all strengthen the sequestration of alkalinity in the deepest ocean, which increases CO2 both by itself and by decreasing whole ocean alkalinity. Increased nutrient consumption in the sub-Antarctic causes as much as an additional 35 ppm CO2 decrease, interacting minimally with the other changes. With its inclusion, the lowest ice age CO2 levels are within reach. These findings may bear on the two-stepped CO2 decrease of the last ice age.

Early Pliocene increase in thermohaline overturning: A precondition for the development of the modern equatorial Pacific cold tongue

Abstract: Unraveling the processes responsible for Earth’s climate transition from an “El Nino–like state” during the warm early Pliocene into a modern‐like “La Nina–dominated state” currently challenges the scientific community. Recently, the Pliocene climate switch has been linked to oceanic thermocline shoaling at ∼3 million years ago along with Earth’s final transition into a bipolar icehouse world. Here we present Pliocene proxy data and climate model results, which suggest an earlier timing of the Pliocene climate switch and a different chain of forcing mechanisms. We show that the increase in North Atlantic meridional overturning circulation between 4.8 and 4.0 million years ago, initiated by the progressive closure of the Central American Seaway, triggered overall shoaling of the tropical thermocline. This preconditioned the turnaround from a warm eastern equatorial Pacific to the modern equatorial cold tongue state about 1 million years earlier than previously assumed. Since ∼3.6–3.5 million years ago, the intensification of Northern Hemisphere glaciation resulted in a strengthening of the trade winds, thereby amplifying upwelling and biogenic productivity at low latitudes.

Recovery of the forest ecosystem in the tropical lowlands of northern Guatemala after disintegration of classic Maya polities

Abstract: We employed paleolimnological methods to investigate tropical forest recovery and soil stabilization that followed abandonment of agricultural systems associated with disintegration of Classic Maya polities ca. A.D. 800–1000. We used lithological, geochemical, magnetic, and palynological data from sediment cores of Lake Peten Itza in the Maya Lowlands of northern Guatemala. Sediment core chronology was developed using radiocarbon dates on terrestrial wood and charcoal fragments. Our results indicate that in the absence of large human populations and extensive farming activities, Peten forests recovered under humid climate conditions within a span of 80–260 yr. Soil stabilization postdates pollen evidence of forest regrowth stratigraphically, and required between 120 and 280 yr. We conclude that the tropical forest ecosystem in the watershed of Lake Peten Itza had been reestablished by the early Postclassic Period (A.D. 1000–1200).

Comment on “On linking climate to Chinese dynastic change: Spatial and temporal variations of monsoonal rain”

Abstract: An article by Zhang et al. questions the interpretation of our oxygen isotope record from Wanxiang Cave and the sediment titanium record from Lake Huguang Maar, and the possible linkage between climate change and Chinese culture. In response, we explain that their approach lacks logical rigor and their major argument is broadly consistent with, rather than contradictory to our original conclusions. We also note that climate-culture relationships similar to those that we observe in China have been observed for other cultures around the world.

Results from IODP Leg 306: Long-term cooling trend in North Atlantic sea-surface temperatures during the last 5 Ma

Abstract: In the early Pliocene global surface temperatures were several degrees warmer than today and ice sheets in the Northern Hemisphere had a limited extent [e.g., Haywood et al., 2005; Zachos et al., 2001]. This changed during the intensification of Northern Hemisphere glaciation (INHG) between 3.4 and 2.5 Ma (with a major step around 2.7 Ma), when global climate cooled and ice sheets in the Northern Hemisphere became more extensive [e.g., Zachos et al., 2001].