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Variability of stalagmite-inferred Indian monsoon precipitation over the past 252,000 y (2015)

Cai, Yanjun ; Fung, Inez Y. ; Edwards, R. Lawrence ; [et al.]

National Academy of Sciences

In: PNAS - Proceedings of the National Academy of Sciences of the United States of America. 2015; 112(10): 2954-2959. Published 2015 Feb 23. doi: 10.1073/pnas.1424035112.

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Publication Date: 2015-02-23

Description: A speleothem δ18O record from Xiaobailong cave in southwest China characterizes changes in summer monsoon precipitation in Northeastern India, the Himalayan foothills, Bangladesh, and northern Indochina over the last 252 kyr. This record is dominated by 23-kyr precessional cycles punctuated by prominent millennial-scale oscillations that are synchronous with Heinrich events in the North Atlantic. It also shows clear glacial–interglacial variations that are consistent with marine and other terrestrial proxies but are different from the cave records in East China. Corroborated by isotope-enabled global circulation modeling, we hypothesize that this disparity reflects differing changes in atmospheric circulation and moisture trajectories associated with climate forcing as well as with associated topographic changes during glacial periods, in particular redistribution of air mass above the growing ice sheets and the exposure of the “land bridge” in the Maritime continents in the western equatorial Pacific.

Print ISSN: 0027-8424

Electronic ISSN: 1091-6490

Topics: Biology , Medicine , Natural Sciences in General

Published by National Academy of Sciences

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Rainfall variations in central Indo-Pacific over the past 2,700 y (2019)

Tan, Liangcheng ; Shen, Chuan-Chou ; Löwemark, Ludvig ; [et al.]

National Academy of Sciences

In: PNAS - Proceedings of the National Academy of Sciences of the United States of America. 2019; 116(35): 17201-17206. Published 2019 Aug 12. doi: 10.1073/pnas.1903167116.

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Publication Date: 2019-08-12

Description: Tropical rainfall variability is closely linked to meridional shifts of the Intertropical Convergence Zone (ITCZ) and zonal movements of the Walker circulation. The characteristics and mechanisms of tropical rainfall variations on centennial to decadal scales are, however, still unclear. Here, we reconstruct a replicated stalagmite-based 2,700-y-long, continuous record of rainfall for the deeply convective northern central Indo-Pacific (NCIP) region. Our record reveals decreasing rainfall in the NCIP over the past 2,700 y, similar to other records from the northern tropics. Notable centennial- to decadal-scale dry climate episodes occurred in both the NCIP and the southern central Indo-Pacific (SCIP) during the 20th century [Current Warm Period (CWP)] and the Medieval Warm Period (MWP), resembling enhanced El Niño-like conditions. Further, we developed a 2,000-y-long ITCZ shift index record that supports an overall southward ITCZ shift in the central Indo-Pacific and indicates southward mean ITCZ positions during the early MWP and the CWP. As a result, the drying trend since the 20th century in the northern tropics is similar to that observed during the past warm period, suggesting that a possible anthropogenic forcing of rainfall remains indistinguishable from natural variability.

Print ISSN: 0027-8424

Electronic ISSN: 1091-6490

Topics: Biology , Medicine , Natural Sciences in General

Published by National Academy of Sciences

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Progressive aridification in East Africa over the last half million years and implications for human evolution (2018)

Owen, R. Bernhart ; Muiruri, Veronica M. ; Lowenstein, Tim K. ; [et al.]

National Academy of Sciences

In: PNAS - Proceedings of the National Academy of Sciences of the United States of America. 2018; 115(44): 11174-11179. Published 2018 Oct 08. doi: 10.1073/pnas.1801357115.

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Publication Date: 2018-10-08

Description: Evidence for Quaternary climate change in East Africa has been derived from outcrops on land and lake cores and from marine dust, leaf wax, and pollen records. These data have previously been used to evaluate the impact of climate change on hominin evolution, but correlations have proved to be difficult, given poor data continuity and the great distances between marine cores and terrestrial basins where fossil evidence is located. Here, we present continental coring evidence for progressive aridification since about 575 thousand years before present (ka), based on Lake Magadi (Kenya) sediments. This long-term drying trend was interrupted by many wet–dry cycles, with the greatest variability developing during times of high eccentricity-modulated precession. Intense aridification apparent in the Magadi record took place between 525 and 400 ka, with relatively persistent arid conditions after 350 ka and through to the present. Arid conditions in the Magadi Basin coincide with the Mid-Brunhes Event and overlap with mammalian extinctions in the South Kenya Rift between 500 and 400 ka. The 525 to 400 ka arid phase developed in the South Kenya Rift between the period when the last Acheulean tools are reported (at about 500 ka) and before the appearance of Middle Stone Age artifacts (by about 320 ka). Our data suggest that increasing Middle- to Late-Pleistocene aridification and environmental variability may have been drivers in the physical and cultural evolution of Homo sapiens in East Africa.

Print ISSN: 0027-8424

Electronic ISSN: 1091-6490

Topics: Biology , Medicine , Natural Sciences in General

Published by National Academy of Sciences

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Multidecadally resolved polarity oscillations during a geomagnetic excursion (2018)

Chou, Yu-Min ; Jiang, Xiuyang ; Liu, Qingsong ; [et al.]

National Academy of Sciences

In: PNAS - Proceedings of the National Academy of Sciences of the United States of America. 2018; 115(36): 8913-8918. Published 2018 Aug 20. doi: 10.1073/pnas.1720404115.

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Publication Date: 2018-08-20

Description: Polarity reversals of the geomagnetic field have occurred through billions of years of Earth history and were first revealed in the early 20th century. Almost a century later, details of transitional field behavior during geomagnetic reversals and excursions remain poorly known. Here, we present a multidecadally resolved geomagnetic excursion record from a radioisotopically dated Chinese stalagmite at 107–91 thousand years before present with age precision of several decades. The duration of geomagnetic directional oscillations ranged from several centuries at 106–103 thousand years before present to millennia at 98–92 thousand years before present, with one abrupt reversal transition occurring in one to two centuries when the field was weakest. These features indicate prolonged geodynamo instability. Repeated asymmetrical interhemispheric polarity drifts associated with weak dipole fields likely originated in Earth’s deep interior. If such rapid polarity changes occurred in future, they could severely affect satellites and human society.

Print ISSN: 0027-8424

Electronic ISSN: 1091-6490

Topics: Biology , Medicine , Natural Sciences in General

Published by National Academy of Sciences

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