Sunday, June 9, 2013

Take It to Print

by Stacy Carolin
We have a paper out (!) And all that text will likely cause a little headache, so I’ve condensed it down to the important talking points –in common tongue-- to bring to the dinner table tonight: 

The author examining a fallen stalagmite. Credit:  Syria Lejau

The new published record was created from stalagmites collected in Gunung Mulu National Park on the island of Borneo, which sits on the equator in the western Pacific. Variability in the isotopic chemistry of the rainwater over our site is directly related to variability in the rainfall amount in the region (see lab-colleague Jessica Moerman's recent study in EPSL). Stalagmites form from rainwater that drips through soil and limestone into cave chambers, and the isotopic chemistry in the rainwater is captured and maintained in the calcite formations over tens of thousands of years. The four stalagmites used in this study grew during most of earth's last ice age (100,000 years ago to 15,000 years ago), and reveal how rainfall amount over Borneo changed (or did not change) in response to various atmospheric and oceanic changes, both global and local, during this period.

Points (1)-(5) on published
Figure 2 --click for larger
(1) Ice-age-cycle global temperature and CO2 variations did not greatly influence rainfall amount over Borneo -- Like the Chinese stalagmite records, the 100,000-year-long Borneo record does not have a "sawtooth" shape famously characteristic in the global temperature, atmospheric CO2, and sea level records spanning several ice age cycles. We conclude then that on these multi-thousand-year timescales, those variables are not directly influencing rainfall amounts over Borneo.

(2) Abrupt sea level drop / sunda shelf exposure did not cause a synchronous shift toward dry (or wet) conditions in Borneo -- Sunda Shelf emergence (shallow continental shelf connecting Borneo to Southeast Asia) has been implicated in shaping western tropical Pacific rainfall amount in previous studies, but we find little correlation between the Borneo records and an index of Sunda Shelf exposure over the last 100,000 years. For example, significant variations in the Borneo records during the earlier glacial interval (90,000-70,000 years ago) bear little resemblance to reconstructed sea level changes, most notably ~76,000-71,000 years ago, when a 60 meter drop in sea level almost doubled the size of the exposed shelf without any corresponding shift in rainfall amount.

(3) Rainfall variability over Borneo closely followed solar radiation cycles over the equator (seasonal solar energy flux increase = rainfall amount increase, and vice versa) -- The Borneo records suggest that solar precessional cycles (meaning change in solar energy flux over the equator due to cyclical change in the orientation of earth's tilted axis) may be the dominant source of Borneo rainfall amount variability on thousand-year timescales. For example, today the tilted axis points the northern hemisphere toward the sun (northern hemisphere summer) when the earth is furthest from the sun in its elliptical orbit, while 12,000 years ago the tilted axis pointed the southern hemisphere toward the sun (northern hemisphere winter) when the earth was furthest from the sun in its elliptical orbit. These cyclical changes on thousand-year timescales vary the seasonal amount of solar energy over Borneo, which the records suggest in turn do drive changes in the amount of rainfall over Borneo.

(4) The Borneo record’s lack of many abrupt climate shifts that are present in almost all northern hemisphere records implies a selective response of Borneo rainfall to high-latitude abrupt climate change forcing -- The dominant paradigm to explain the famous global millennial-scale climate shifts first discovered in the Greenland ice cores is that they are driven from the North Atlantic region, either from weakening of the Atlantic ocean circulation or from a dramatic albedo change due to sea ice cover, which in turn shifts the northern hemisphere to cooler/drier conditions and makes the southern hemisphere warmer/wetter. The absence of several of these abrupt climate change signals in the Borneo record represents a clear challenge to our understanding of how the tropical Pacific is involved in the mechanism(s) that forced abrupt climate change during earth’s last glacial state.

(5) The largest millennial-scale anomaly in the 100,000-year Borneo record is coincident with the Toba super eruption -- The Toba eruption was the largest volcanic eruption in the past 2.5 million years and climate responses to the eruption are uncertain. Many anthropologists believe that the volcanic winter following the super-eruption decimated most “modern human” populations at that time, and caused a bottleneck in human evolution. The abrupt shift toward dry conditions over Borneo (within age error of the volcanic eruption) that then lasted for a millennium is interestingly mirrored in the Chinese stalagmite record. Further studies are needed to determine if these events are at all related.

Thanks to Kim, Jess, Jessica, Syria, Jenny, and all the rest for helping put this beast together, from field to print! What an experience!

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