Possible expression of the 4.2 kyr event in Madagascar and the south-east African monsoon
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Abstract. The 4.2 kyr event is regarded as one of the largest and best documented abrupt climate disturbances of the Holocene. Drying across the Mediterranean and Middle East is well established and is linked to societal transitions in the Akkadian, Egyptian and Harappan civilizations. Yet the impacts of this regional drought are often extended to other regions and sometimes globally. In particular, the nature and spatial extent of the 4.2 kyr event in the tropics have not been established. Here, we present a new stalagmite stable isotope record from Anjohikely, northwest Madagascar. Growing between 5 and 2 kyr BP, stalagmite AK1 shows a hiatus between 4.32 and 3.83 kyr BP, replicating a hiatus in another stalagmite from nearby Anjohibe, and therefore indicating a significant drought around the time of the 4.2 kyr event. This result is the opposite to wet conditions at 8.2 kyr BP, suggesting fundamentally different forcing mechanisms. Elsewhere in the south-east African monsoon domain dry conditions are also recorded in sediment cores in Lake Malawi and Lake Masoko and the Taros Basin on Mauritius. However, at the peripheries of the monsoon domain, drying is not observed. At the northern (equatorial East Africa) and eastern (Rodrigues) peripheries, no notable event is record. At the southern periphery a wet event is recorded in stalagmites at Cold Air Cave and sediment cores at Lake Muzi and Mkhuze Delta. The spatial pattern is largely consistent with the modern rainfall anomaly pattern associated with weak Mozambique Channel Trough and a northerly austral summer Inter Tropical Convergence Zone position. Within age error, the observed peak climate anomalies are consistent with the 4.2 kyr event. However, outside Madagascar, regional hydrological change is consistently earlier than a 4.26 kyr BP event onset. Gradual hydrological change frequently begins at 4.6 kyr BP, raising doubt as to whether any coherent regional hydrological change is merely coincident with the 4.2 kyr event rather than part of a global climatic anomaly.Keywords:
Stalagmite
Hiatus
Trough (economics)
Intertropical Convergence Zone
East Asian Monsoon
Three 230Th dating results and 236 oxygen isotope data of a 1.2-m-long stalagmite were collected from Shanbao Cave in Shennongjia of Hubei Province.Within the dating uncertainty,an astronomical tuning was applied to reconstruct an East Asian summer monsoon history from 571 to 476 kaBP.Our record spans four precessional cycles,with an average amplitude of 4,which suggests East Asian monsoon variability was still paced by insolation in this period.A similar pattern to the last deglacial was found during the transition of MIS 14/13.A series of millennial climate events were superposed on the precessional trend of East Asian summer monsoon,indicating an inherent frequency selfsustained in the global climate system,which may be an interaction of ocean,land and atmosphere.
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East Asian Monsoon
Little ice age
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Abstract. Petrographic features, mineralogy, and stable isotopes from two stalagmites collected from Anjohibe and Anjokipoty Cave allow distinction of three intervals of the Holocene in northwestern Madagascar. The Malagasy early Holocene interval (between ca. 9.8 and 7.8 ka) was wet, and vegetation changes seem to have been controlled by changes in climate. The Malagasy late Holocene interval (after ca. 1.6 ka) also records evidence of wet conditions, but changes in vegetation were influenced by anthropogenic effects, as suggested by the stalagmite δ13C shift. The Malagasy middle Holocene interval seems to be characterized by drier conditions, relative to the early and late Holocene. The alternating wet/dry/wet conditions in northwestern Madagascar during each of these Holocene intervals could be linked to the long-term migration of the Inter-Tropical Convergence Zone (ITCZ). Higher southern hemisphere (SH) insolation and globally colder conditions drove the ITCZ's mean position further south, bringing more rainfall to northwestern Madagascar. This condition was favorable for stalagmite deposition. In contrast, higher northern hemisphere (NH) insolation and globally warmer conditions displaced the ITCZ further north, bringing less rainfall to northwestern Madagascar. This condition was not favorable for stalagmite deposition. The linkage between global cooling and wet conditions in regions of the SH, in response to the southward migration of the ITCZ, is further exemplified at centennial scale by the negative δ18O and δ13C values in northwestern Madagascar during the 8.2 ka cold event when the Atlantic Meridional Overturning Circulation (AMOC) weakened. Weakening of the AMOC led to an enhanced temperature gradient between the two hemispheres, i.e. cold NH and warm SH, shifting the mean position of the ITCZ further south. This brought wet conditions in the SH monsoon regions, such as northwestern Madagascar, and dry conditions in the NH monsoon regions, including the Asian Monsoon and the East Asian Summer Monsoon. This climatic relationship is useful to test for climate models that are used to predict changes in future climate.
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Intertropical Convergence Zone
Paleoclimatology
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Abstract The South American Monsoon System (SAMS) is generally considered to be highly sensitive to Northern Hemisphere (NH) temperature variations on multi-centennial timescales. The direct influence of solar forcing on moisture convergence in global monsoon systems on the other hand, while well explored in modeling studies, has hitherto not been documented in proxy data from the SAMS region. Hence little is known about the sensitivity of the SAMS to solar forcing over the past millennium and how it might compete or constructively interfere with NH temperature variations that occurred primarily in response to volcanic forcing. Here we present a new annually-resolved oxygen isotope record from a 1500-year long stalagmite recording past changes in precipitation in the hitherto unsampled core region of the SAMS. This record details how solar variability consistently modulated the strength of the SAMS on centennial time scales during the past 1500 years. Solar forcing, besides the previously recognized influence from NH temperature changes and associated Intertropical Convergence Zone (ITCZ) shifts, appears as a major driver affecting SAMS intensity at centennial time scales.
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Forcing (mathematics)
Centennial
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Cave sediments, especially stalagmites, have been providing absolute dated climate records that can extend from the present to over 500,000 years ago. Based on the reconstructed temperature time series, a comprehensive overview of the climatic differences and similarities between the Indian and the East Asian Monsoon regions of China over the last millennium is presented. Evidence from accurately dated and high-resolution records including stalagmites, ice cores and tree rings show that there was a "Medieval Warm Period" (around 1000 to 1400 AD) in north and east China where climate is dominated by the East Asian monsoon; whilst no such interval is evident in the records including stalagmites and ice cores from southwest China where climate is dominated by the Indian monsoon. However, both regions underwent a significant cooling during the Little Ice Age (around the mid 1500s to the 1800s). The result achieved here may allow a possibility of distinguishing the boundary between Indian monsoon and East Asian monsoon regions over the last millennium with increase of climate records, especially stalagmites that are mostly suitable for accurate U/Th dating and/or lamina counting.
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This study focuses on speleothems from two caves in Madagascar.Several types of analysis are performed including stable isotopes, laminae, and mineralogy, each of which is anchored using U-Th dates.The age models appear robust (although an adequate discussion of age determinations and age model calculations is lacking) but there are several problems.First, the time slices spanned by these stalagmites are quite short, being punctuated by long hiatuses.As a result, the larger context of this record is difficult to identify.Second, I am not convinced of the corrections for differential fractionation between calcite and aragonite d13C values.And associated with this is my concern that there may be microscopically intermingled aragonite and
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Speleothem
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Stalagmite
East Asian Monsoon
Speleothem
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Indian Monsoon and East Asian Monsoon are two sub-systems of Asian Monsoon.A common pattern between the two monsoons at the seasonal and orbital time scales was formulated from the meteorological data and geological records.However,the relationship of the two sub-systems on the inter-annual and inter-decadal scales remains quite obscure.In this study,high-resolution oxygen isotope compositions of two laminated speoleothems from the Defore Cave of Oman and the Heshang Cave of China were compared with each other to detect the link between the Indian Monsoon and the East Asian Monsoon.δ18O of Defore stalagmite is characteristic of the Indian Monsoon while δ18O of Heshang stalagmite is an indicator of the East Asian Monsoon.Two stalagmite records show similar structures for the last 780 years on the decadal scale,implying the asynchronous variation of the Indian Monsoon and the East Asian Monsoon.
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East Asian Monsoon
δ18O
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Abstract. Petrographic features, mineralogy, and stable isotopes from two stalagmites, ANJB-2 and MAJ-5, respectively from Anjohibe and Anjokipoty caves, allow distinction of three intervals of the Holocene in NW Madagascar. The Malagasy early Holocene (between ca. 9.8 and 7.8 ka) and late Holocene (after ca. 1.6 ka) intervals (MEHI and MLHI, respectively) record evidence of stalagmite deposition. The Malagasy middle Holocene interval (MMHI, between ca. 7.8 and 1.6 ka) is marked by a depositional hiatus of ca. 6500 years. Deposition of these stalagmites indicates that the two caves were sufficiently supplied with water to allow stalagmite formation. This suggests that the MEHI and MLHI intervals may have been comparatively wet in NW Madagascar. In contrast, the long-term depositional hiatus during the MMHI implies it was relatively drier than the MEHI and the MLHI. The alternating wet–dry–wet conditions during the Holocene may have been linked to the long-term migrations of the Intertropical Convergence Zone (ITCZ). When the ITCZ's mean position is farther south, NW Madagascar experiences wetter conditions, such as during the MEHI and MLHI, and when it moves north, NW Madagascar climate becomes drier, such as during the MMHI. A similar wet–dry–wet succession during the Holocene has been reported in neighboring locations, such as southeastern Africa. Beyond these three subdivisions, the records also suggest wet conditions around the cold 8.2 ka event, suggesting a causal relationship. However, additional Southern Hemisphere high-resolution data will be needed to confirm this.
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Intertropical Convergence Zone
Paleoclimatology
Hiatus
Holocene climatic optimum
Speleothem
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Stalagmite
δ18O
East Asian Monsoon
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