Southern Hemisphere controls on ITCZ variability in southwest Madagascar over the past 117,000 years

Migration of the inter-tropical convergence zone, driven by changes in seasonal insolation and high northern latitude temperatures, is the primary control on tropical rainfall on geologic timescales. We test this paradigm using the timing of growth of stalagmites from southwest Madagascar to infer the timing of expansion of the ITCZ to the south at its southern limit. Over the past 117 ky, speleothems grew in the study area primarily when two conditions are met: summer insolation greater than the mean and relatively high Southern Hemisphere temperatures as indicted by maxima in Antarctic ice core oxygen isotope ratios. We observe little influence of Northern Hemisphere, millennial scale temperature variability on the pluvial periods. Further, we observe periods during which the ITCZ simultaneously expands or contracts in both hemispheres. Because Antarctic isotope maxima are periods of increased atmospheric CO2, our results have implications for how tropical rainfall in the Southern Hemisphere might respond to global warming

Tropical Indian Ocean basin hydroclimate at the Mid- to Late-Holocene transition and the double drying hypothesis

The spatial pattern of Holocene climate anomalies is crucial to determining the mechanisms of change, distinguishing between unforced and forced climate variability, and understanding potential impacts on past and future human societies. The 4.2 ka event is often regarded as one of the largest and best documented abrupt climate disturbances of the Holocene. Yet outside the data-rich Northern Hemisphere mid-latitudes, the global pattern of climate anomalies is uncertain. In this study we investigate the spatial and temporal variability of the tropical Indian Ocean hydroclimate at the Mid- to Late-Holocene transition. We conducted Monte-Carlo principal component analysis, considering full age uncertainty, on ten high-resolution, precisely dated paleohydroclimate records from around the tropical Indian Ocean basin, all growing continuously or almost continuously between 5 and 3 kyr BP. The results indicate the dominant mode of variability in the region was a drying between 3.97 kyr BP (±0.08 kyr standard error) and 3.76 kyr BP (±0.07 kyr standard error) with dry conditions lasting for an additional 300 years in some records, and a permanent change in others. This drying in PC1, which we interpret as a proxy of summer monsoon variability, fits with a previously recognised tropic wide change in hydroclimate around 4.0 kyr BP. An abrupt event from 4.2 to 3.9 kyr BP is seen locally in individual records but lacks regional coherence. A lack of apparent 4.2 ka event in tropical Indian Ocean hydroclimate has ramifications for climate variability in the Indus valley, and for the Harappan civilization. Through a comparison of existing Indian subcontinent paleoclimate records, upstream climatic variability in the Indian Summer Monsoon and winter Westerly Disturbances source regions, and modern climatology, we present the “Double Drying hypothesis”. A winter rainfall drying between 4.2 and 3.9 kyr BP was followed by a summer rainfall drying between 3.97 kyr BP and at least 3.4 kyr BP. The Double Drying hypothesis provides more detailed climatic context for the Harappan civilization, resolves the cropping paradox, and fits the spatial-temporal pattern of urban abandonment. The consequences for the new Mid- to Late-Holocene Global Boundary Stratotype Section and Point in a stalagmite from Meghalaya are explored

Hydroclimate variability in the Madagascar and Southeast African summer monsoons at the Mid- to Late-Holocene transition

The 4.2 ka event at the Mid- to Late-Holocene transition is often regarded as one of the largest and best documented abrupt climate disturbances of the Holocene. The event is most clearly manifested in the Mediterranean and Middle East as a regional dry anomaly beginning abruptly at 4.26 kyr BP and extending until 3.97 kyr BP. 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 ka event in the tropics have not been established. Here, we present a new stalagmite stable isotope record from Anjohikely, northwest Madagascar. Growing between 5.22 and 2.00 kyr BP, stalagmite AK1 shows a hiatus between 4.31 and 3.93 kyr BP (±40 and ± 35 yrs), replicating a hiatus in another stalagmite from nearby Anjohibe, and therefore indicating a significant drying at the Mid- to Late-Holocene transition. This result is the opposite to wet conditions at the 8.2 ka event, suggesting fundamentally different forcing mechanisms. Dry conditions are also recorded in sediment cores in Lake Malawi, Lake Masoko and the Tatos Basin on Mauritius, also in the southeast African monsoon domain. However, no notable event is recorded at the northern (equatorial East Africa) and eastern (Rodrigues) peripheries of the monsoon domain, while a wet event is recorded in sediment cores at Lake Muzi and Mkhuze Delta to the south. The spatial pattern is largely consistent with the modern rainfall anomaly pattern associated a with weak Mozambique Channel Trough and a northerly austral summer Intertropical Convergence Zone position. Within age error, the observed peak climate anomalies overlap with the 4.2 ka event. However regional hydrological change consistently begins earlier than a 4.26 kyr BP event onset. Gradual hydrological change frequently begins around 4.5 kyr BP, raising doubt as to whether any coherent regional hydrological change is merely coincident with the 4.2 ka event or part of a global climatic anomaly

Comparing the paleoclimates of northwestern and southwestern Madagascar during the late Holocene: Implications for the role of climate in megafaunal extinction.

The relative importance of climate and humans in the disappearance of the Malagasy megafauna remains under debate. Data from southwestern Madagascar imply aridifcation contributed substantially to the late Holocene decline of the megafauna (the Aridifcation Hypothesis). Evidence for aridifcation includes carbon isotopes from tree rings, lacustrine charcoal concentrations and pollen assemblages, and changes in fossil vertebrate assemblages indicative of a local loss of pluvial conditions. In contrast, speleothem records from northwestern Madagascar suggest that megafaunal decline and habitat change resulted primarily from human activity including agropastoralism (the Subsistence Shift Hypothesis). Could there have been contrasting mechanisms of decline in different parts of Madagascar? Or are we lacking the precisely dated, high resolution records needed to fully understand the complex processes behind megafaunal decline? Reconciling these contrasting hypotheses requires additional climate records from southwestern Madagascar. We recovered a stalagmite (AF2) from Asafora Cave in the spiny thicket ecoregion, ~10 km from the southwest coast and just southeast of the Velondriake Marine Reserve. U-series and 14C dating of samples taken from the core of this stalagmite provide a highly precise chronology of the changes in hydroclimate and vegetation in this region over the past 3000 years. Speleothem stable oxygen and carbon isotope analyses provide insight into past rainfall variability and vegetation changes respectively. We compare these records with those for a stalagmite (AB2) from Anjohibe Cave in northwestern Madagascar. Lastly, odds ratio analyses of radiocarbon dates for extinct and extant subfossils allow us to describe and compare the temporal trajectories of megafaunal decline in the southwest and the northwest. Combined, these analyses allow us to test the Aridifcation Hypothesis for megafaunal extinction. The trajectories of megafaunal decline differed in northwestern and southwestern Madagascar. In the southwest, unlike the northwest, there is no evidence of decoupling of speleothem stable carbon and oxygen isotopes. Instead, habitat changes in the southwest were largely related to variation in hydroclimate (including a prolonged drought). The megafaunal collapse here occurred in tandem with the drought, and agropastoralism likely contributed to that demise only after the megafauna had already suffered drought-related population reduction. Our results offer some support for the Aridifcation Hypothesis, but with three caveats: frst, that there was no island-wide aridifcation; second, that aridifcation likely impacted megafaunal decline only in the driest parts of Madagascar; and third, that aridifcation was not the sole factor promoting Comparing the paleoclimates of northwestern and southwestern Madagascar during the late Holocene: Implications for the role of climate in megafaunal extinction Faina et al.: Comparing the paleoclimates of northwestern and southwestern Madagascar 109 megafaunal decline even in the dry southwest. A number of megafaunal species survived the prolonged drought of the first millennium, and then likely succumbed to the activities of agropastoralists

USING SPELEOTHEM FLUID INCLUSION ISOTOPIC COMPOSITION TO RECONSTRUCT THE TEMPERATURE AND HYDROLOGIC RECORD OF THE LAST 60,000 YEARS IN SOUTHWESTERN MADAGASCAR

The southern hemisphere tropical climate response to glacial/interglacial climate transitions is poorly understood because there are relatively few quantitative reconstructions of climate in these regions. To gain insight into tropical climate during the last glacial period and over the transition into the Holocene we measured the stable isotope ratios of fluid inclusions from U/Th dated speleothems from southwestern Madagascar ( approximately 24 degrees S). These data provide estimates of the isotopic composition of precipitation and, when combined with measurements of the isotopic composition of speleothem calcite, paleotemperatures for the last approximately 60,000 years. Preliminary results show the delta (super 18) O (sub fi) and delta D (sub fi) composition of fluid inclusion water falls within error (95%) of the modern local meteoric water line, based on station data collected from the capital, Antananarivo. One exception is a modern sample, which falls above the line. Other recent samples have delta (super 18) O (sub fi) values from -4.13 ppm to -5.93 ppm and delta D (sub fi) values from -21.41 ppm to -28.69 ppm, while Holocene delta (super 18) O (sub fi) and delta D (sub f) i values are more enriched, -3.86 ppm and -19.37 ppm. The LGM has a range of delta (super 18) O (sub fi) and delta D (sub fi) values from -3.13 to -6.09 ppm, and -21.37 to -24.97 ppm respectively. The rest of our record has even greater variability with a range of delta (super 18) O (sub fi) and delta D (sub fi) values from -2.11 to -5.58 ppm, and -8.13 to -37.51 ppm respectively. Combining the delta (super 18) O of speleothem carbonate along with the delta (super 18) O of fluid inclusions and assuming equilibrium precipitation, we can calculate cave calcification temperatures, which are generally considered to yield MAAT. Initial data suggest approximately 10 degrees C of warming from the last glacial period to the Holocene in southwestern Madagascar. The onset of warming, approximately 20 kya agrees with organic biomarker temperature records from East Africa lakes, albeit the degree of warming is greater in our Madagascar record, possibly due to being situated further south or differences in what the proxies represent (i.e. lake surfaces vs cave interiors). Future work will focus on additional fluid inclusion measurements to extend this record and provide higher resolution estimates of southern hemisphere tropical temperatures and hydrologic changes from the last glacial period to today

Comparing the paleoclimates of northwestern and southwestern Madagascar during the late Holocene: Implications for the role of climate in megafaunal extinction.

The relative importance of climate and humans in the disappearance of the Malagasy megafauna remains under debate. Data from southwestern Madagascar imply aridifcation contributed substantially to the late Holocene decline of the megafauna (the Aridifcation Hypothesis). Evidence for aridifcation includes carbon isotopes from tree rings, lacustrine charcoal concentrations and pollen assemblages, and changes in fossil vertebrate assemblages indicative of a local loss of pluvial conditions. In contrast, speleothem records from northwestern Madagascar suggest that megafaunal decline and habitat change resulted primarily from human activity including agropastoralism (the Subsistence Shift Hypothesis). Could there have been contrasting mechanisms of decline in different parts of Madagascar? Or are we lacking the precisely dated, high resolution records needed to fully understand the complex processes behind megafaunal decline? Reconciling these contrasting hypotheses requires additional climate records from southwestern Madagascar. We recovered a stalagmite (AF2) from Asafora Cave in the spiny thicket ecoregion, ~10 km from the southwest coast and just southeast of the Velondriake Marine Reserve. U-series and 14C dating of samples taken from the core of this stalagmite provide a highly precise chronology of the changes in hydroclimate and vegetation in this region over the past 3000 years. Speleothem stable oxygen and carbon isotope analyses provide insight into past rainfall variability and vegetation changes respectively. We compare these records with those for a stalagmite (AB2) from Anjohibe Cave in northwestern Madagascar. Lastly, odds ratio analyses of radiocarbon dates for extinct and extant subfossils allow us to describe and compare the temporal trajectories of megafaunal decline in the southwest and the northwest. Combined, these analyses allow us to test the Aridifcation Hypothesis for megafaunal extinction. The trajectories of megafaunal decline differed in northwestern and southwestern Madagascar. In the southwest, unlike the northwest, there is no evidence of decoupling of speleothem stable carbon and oxygen isotopes. Instead, habitat changes in the southwest were largely related to variation in hydroclimate (including a prolonged drought). The megafaunal collapse here occurred in tandem with the drought, and agropastoralism likely contributed to that demise only after the megafauna had already suffered drought-related population reduction. Our results offer some support for the Aridifcation Hypothesis, but with three caveats: frst, that there was no island-wide aridifcation; second, that aridifcation likely impacted megafaunal decline only in the driest parts of Madagascar; and third, that aridifcation was not the sole factor promoting Comparing the paleoclimates of northwestern and southwestern Madagascar during the late Holocene: Implications for the role of climate in megafaunal extinction Faina et al.: Comparing the paleoclimates of northwestern and southwestern Madagascar 109 megafaunal decline even in the dry southwest. A number of megafaunal species survived the prolonged drought of the first millennium, and then likely succumbed to the activities of agropastoralists

Zonal control on Holocene precipitation in northwestern Madagascar based on a stalagmite from Anjohibe

Abstract The Malagasy Summer Monsoon is an important part of the larger Indian Ocean and tropical monsoon region. As the effects of global warming play out, changes to precipitation in Madagascar will have important ramifications for the Malagasy people. To help understand how precipitation responds to climate changes we present a long-term Holocene speleothem record from Anjohibe, part of the Andranoboka cave system in northwestern Madagascar. To date, it is the most complete Holocene record from this region and sheds light on the nature of millennial and centennial precipitation changes in this region. We find that over the Holocene, precipitation in northwestern Madagascar is actually in phase with the Northern Hemisphere Asian monsoon on multi-millennial scales, but that during some shorter centennial-scale events such as the 8.2 ka event, Anjohibe exhibits an antiphase precipitation signal to the Northern Hemisphere. The ultimate driver of precipitation changes across the Holocene does not appear to be the meridional migration of the monsoon. Instead, zonal sea surface temperature gradients in the Indian Ocean seem to play a primary role in precipitation changes in northwestern Madagascar

Comparing the paleoclimates of northwestern and southwestern Madagascar during the late Holocene: Implications for the role of climate in megafaunal extinction.

The relative importance of climate and humans in the disappearance of the Malagasy megafauna remains under debate. Data from southwestern Madagascar imply aridifcation contributed substantially to the late Holocene decline of the megafauna (the Aridifcation Hypothesis). Evidence for aridifcation includes carbon isotopes from tree rings, lacustrine charcoal concentrations and pollen assemblages, and changes in fossil vertebrate assemblages indicative of a local loss of pluvial conditions. In contrast, speleothem records from northwestern Madagascar suggest that megafaunal decline and habitat change resulted primarily from human activity including agropastoralism (the Subsistence Shift Hypothesis). Could there have been contrasting mechanisms of decline in different parts of Madagascar? Or are we lacking the precisely dated, high resolution records needed to fully understand the complex processes behind megafaunal decline? Reconciling these contrasting hypotheses requires additional climate records from southwestern Madagascar. We recovered a stalagmite (AF2) from Asafora Cave in the spiny thicket ecoregion, ~10 km from the southwest coast and just southeast of the Velondriake Marine Reserve. U-series and 14C dating of samples taken from the core of this stalagmite provide a highly precise chronology of the changes in hydroclimate and vegetation in this region over the past 3000 years. Speleothem stable oxygen and carbon isotope analyses provide insight into past rainfall variability and vegetation changes respectively. We compare these records with those for a stalagmite (AB2) from Anjohibe Cave in northwestern Madagascar. Lastly, odds ratio analyses of radiocarbon dates for extinct and extant subfossils allow us to describe and compare the temporal trajectories of megafaunal decline in the southwest and the northwest. Combined, these analyses allow us to test the Aridifcation Hypothesis for megafaunal extinction. The trajectories of megafaunal decline differed in northwestern and southwestern Madagascar. In the southwest, unlike the northwest, there is no evidence of decoupling of speleothem stable carbon and oxygen isotopes. Instead, habitat changes in the southwest were largely related to variation in hydroclimate (including a prolonged drought). The megafaunal collapse here occurred in tandem with the drought, and agropastoralism likely contributed to that demise only after the megafauna had already suffered drought-related population reduction. Our results offer some support for the Aridifcation Hypothesis, but with three caveats: frst, that there was no island-wide aridifcation; second, that aridifcation likely impacted megafaunal decline only in the driest parts of Madagascar; and third, that aridifcation was not the sole factor promoting Comparing the paleoclimates of northwestern and southwestern Madagascar during the late Holocene: Implications for the role of climate in megafaunal extinction Faina et al.: Comparing the paleoclimates of northwestern and southwestern Madagascar 109 megafaunal decline even in the dry southwest. A number of megafaunal species survived the prolonged drought of the first millennium, and then likely succumbed to the activities of agropastoralists