The glacial-interglacial monsoon recorded by stalagmites from southwest Sulawesi, Indonesia

Climate variability in the tropics, and monsoons in particular, are critical to transporting moisture and heat around the earth. Deep atmospheric convection over the Indo-Pacific Warm Pool (IPWP) supplies heat and energy to two of the largest atmospheric circulation systems, the meridional Hadley Cell and zonal Indo-Pacific Walker Circulation. These convection centers are key components of the Australasian monsoon. Thus, the IPWP plays a fundamental role in global atmospheric circulation and hydrology. Paleoclimate records of IPWP climate have provided insight into the behavior of these major convective systems; however, significant differences between the reconstructions reveal a non-uniform response to various climate forcings. Recent modeling studies indicate that IPWP hydroclimate and convection are exceptionally sensitive to sea level and the exposure of the Sunda and Sahul shelves over glacial-interglacial (G-I) cycles, but the proxy evidence has not consistently supported this finding. Our study site, in southwest Sulawesi, is located within the IPWP and maintains a climatology dominated by the Australian-Indonesian summer monsoon (AISM). This site is an excellent location for tracking changes in IPWP climate variability and AISM strength through time. Stalagmite geochemical records have the potential to serve as precise monitors of past climatic and environmental change and can be absolute-dated to resolve terrestrial climate change on decadal to orbital timescales. We present eight new multi-proxy stalagmite records from the Maros karst in southwest Sulawesi, which collectively span 386–127 ky BP. The antiquity of the material inspired the design of a U-Th dating procedure using ultraviolet and transparency imaging to select the cleanest material, as well as geochemical assessment of U-Th data to construct robust and optimized chronologies. We investigated different age-depth modeling approaches and identified the Bayesian accumulation algorithm (BACON), coupled with tie-point optimization, as the ideal age-modeling technique for this work. Trace elements (Mg/Ca and Sr/Ca) in the Sulawesi stalagmites were assessed and found to derive primarily from the limestone host rock. Strong co-variation between Mg/Ca and Sr/Ca in slow-growing stalagmites demonstrates their sensitivity to prior calcite precipitation (PCP), and thus hydrological changes, whereas Mg/Ca and Sr/Ca in faster growing stalagmites appear to be influenced by additional factors. Using the hydrologically sensitive Mg/Ca data for two highly resolved (40–180 year-resolution), slow-growing stalagmites, we were able to test and confirm the reliability of stalagmite δ18O as a proxy for rainfall amount at our site. Coupled stalagmite δ18O and Mg/Ca records are particularly useful for investigating G-I changes in IPWP hydroclimate. The stalagmite δ18O record reveals coherent climatic features over G-I transitions. We focus on AISM variability during three periods of rapid global warming, glacial termination TIV (~340 ky BP) and both phases of TIII (TIII ~248 ky BP and TIIIa ~217 ky BP). Stalagmite δ18O and Mg/Ca data show rapid increases in rainfall during glacial terminations and wet interglacials. Glacial terminations are each characterized by an abrupt ~3‰ decrease in stalagmite δ18O that coincides with sea level rise, flooding of the Sunda and Sahul shelves, and strengthening of the AISM. Importantly, the good agreement of the δ18O and Mg/Ca records over G-I transitions demonstrates that southwest Sulawesi stalagmite δ18O is a reliable indicator of rainfall amount. The results show that, on G-I timescales, the strength of the AISM is most sensitive to the impact of changing sea level on the regional distribution of land and shallow-ocean. The new data, alongside an existing 40 ky record for southwest Sulawesi, provide eight transitional stages associated with G-I sea level change and reveal strong evidence for a shelf flooding/exposure threshold of approximately -65 ±9 m. Considered together, both glacial inceptions and glacial terminations imply a sea level threshold driving the AISM between two primary modes of intensity (‘interglacial’ and ‘glacial’). Precession-scale (23 kyr) monsoon variability associated with boreal autumn insolation at the equator is superimposed on these massive sea level driven shifts in AISM strength, indicating an underlying sensitivity of the AISM to the Walker Circulation’s influence on IPWP convection. When compared to stalagmite δ18O records for China, we find that AISM strengthening at glacial terminations consistently coincides with the final stages of weak monsoon intervals in the East Asian summer monsoon record, a key feature of global deglaciation. This observation, coupled with the inferred -65 m sea level threshold driving AISM strength, suggests that reinvigoration of the AISM, and thus IPWP convection, plays an important role in achieving and sustaining interglacial conditions. Although still in its infancy, this work provides critical insight into IPWP behavior over multiple G-I cycles, prompting new questions and inspiring collaboration with the paleoclimate modeling community

Western Pacific hydroclimate linked to global climate variability over the past two millennia

Interdecadal modes of tropical Pacific ocean-atmosphere circulation have a strong influence on global temperature, yet the extent to which these phenomena influence global climate on multicentury timescales is still poorly known. Here we present a 2,000-year, multiproxy reconstruction of western Pacific hydroclimate from two speleothem records for southeastern Indonesia. The composite record shows pronounced shifts in monsoon rainfall that are antiphased with precipitation records for East Asia and the central-eastern equatorial Pacific. These meridional and zonal patterns are best explained by a poleward expansion of the Australasian Intertropical Convergence Zone and weakening of the Pacific Walker circulation (PWC) between ~1000 and 1500 CE Conversely, an equatorward contraction of the Intertropical Convergence Zone and strengthened PWC occurred between ~1500 and 1900 CE. Our findings, together with climate model simulations, highlight the likelihood that century-scale variations in tropical Pacific climate modes can significantly modulate radiatively forced shifts in global temperature

Multi-proxy validation of glacial-interglacial rainfall variations in southwest Sulawesi

Speleothem δ18O is widely used as a proxy for rainfall amount in the tropics on glacial-interglacial to interannual scales. However, uncertainties in the interpretation of this renowned proxy pose a vexing problem in tropical paleoclimatology. Here, we present paired multi-proxy geochemical measurements for stalagmites from southwest Sulawesi, Indonesia, and confirm changes in rainfall amount across ice age terminations. Collectively, the stalagmites span two glacial-interglacial transitions from ~380,000 to 330,000 and 230,000 to 170,000 years ago. Mg/Ca in the slow-growing stalagmites is affected by water moving through the karst and prior calcite precipitation, making it a good proxy for changes in local rainfall. When paired, Mg/Ca and δ18O corroborate prominent shifts from drier glacials to wetter interglacials in the core of the Australasian monsoon domain. These shifts in rainfall occur 4,000-7,000 years later than glacial-interglacial increases in global temperature and the associated response of Sulawesi vegetation, determined by speleothem δ13C

Coral geochemical response to uplift in the aftermath of the 2005 Nias–Simeulue earthquake

On 28 March 2005, the Indonesian islands of Nias and Simeulue experienced a powerful Mw 8.6 earthquake and coseismic uplift and subsidence. In areas of coastal uplift (up to ~ 2.8 m), fringing reef coral communities were killed by exposure, while deeper corals that survived were subjected to habitats with altered runoff, sediment and nutrient regimes. Here we present time-series (2000–2009) of Mn/Ca, Y/Ca and Ba/Ca variability in massive Porites corals from Nias to assess the environmental impact of a wide range of vertical displacement (+ 2.5 m to − 0.4 m). High-resolution LA-ICP-MS measurements show that skeletal Mn/Ca increased at uplifted sites, regardless of reef type, indicating a post-earthquake increase in suspended sediment delivery. Transient and/or long-term increases in skeletal Y/Ca at all uplift sites support the idea of increased sediment delivery. Coral Mn/Ca and Ba/Ca in lagoonal environments highlight the additional influences of reef bathymetry, wind-driven sediment resuspension, and phytoplankton blooms on coral geochemistry. Together, the results show that the Nias reefs adapted to fundamentally altered hydrographic conditions. We show how centuries of repeated subsidence and uplift during great-earthquake cycles along the Sunda megathrust may have shaped the modern-day predominance of massive scleractinian corals on the West Sumatran reefs

Western Pacific hydroclimate linked to global climate variability over the past two millennia

International audienceInterdecadal modes of tropical Pacific ocean-atmosphere circulation have a strong influence on global temperature, yet the extent to which these phenomena influence global climate on multicentury timescales is still poorly known. Here we present a 2,000-year, multiproxy reconstruction of western Pacific hydroclimate from two speleothem records for southeastern Indonesia. The composite record shows pronounced shifts in monsoon rainfall that are antiphased with precipitation records for East Asia and the central-eastern equatorial Pacific. These meridional and zonal patterns are best explained by a poleward expansion of the Australasian Intertropical Convergence Zone and weakening of the Pacific Walker circulation (PWC) between similar to 1000 and 1500 CE Conversely, an equatorward contraction of the Intertropical Convergence Zone and strengthened PWC occurred between similar to 1500 and 1900 CE. Our findings, together with climate model simulations, highlight the likelihood that century-scale variations in tropical Pacific climate modes can significantly modulate radiatively forced shifts in global temperature

The historical impact of anthropogenic air-borne sulphur on the Pleistocene rock art of Sulawesi

The Maros-Pangkep karst in southwest Sulawesi, Indonesia, contains some of the world’s oldest rock art. However, the Pleistocene images survive only as weathered patches of pigment on exfoliated limestone surfaces. Salt efflorescence underneath the case-hardened limestone substrate causes spall-flaking, and it has been proposed that the loss of artwork has accelerated over recent decades. Here, we utilise historical photographs and superposition constraints to show that the bulk of the damage was present before 1950 CE, and describe the role of anthropogenic sulphur emissions in promoting gypsum-salt efflorescence and rock art decay. The rock art shelters have been exposed to domestic fire-use and intensive rice cultivation with post-harvest burning of straw for hundreds (if not thousands) of years, both of which release chemically reactive sulphur oxides for gypsum formation, with cumulative effects. Analysis of time-lapse photography indicates that the rate of rock art loss may be on the decline, consistent with the history of fire-use in southwest Sulawesi. At present, vandalism and sulphur emissions from diesel-powered traffic and cement-based infrastructure development constitute localised threats. Our findings indicate that there are grounds for being cautiously optimistic that targeted conservation measures will ensure the longevity of some of our oldest artistic treasures

Western Pacific hydroclimate linked to global climate variability over the past two millennia

Interdecadal modes of tropical Pacific ocean-atmosphere circulation have a strong influence on global temperature, yet the extent to which these phenomena influence global climate on multicentury timescales is still poorly known. Here we present a 2,000-year, multiproxy reconstruction of western Pacific hydroclimate from two speleothem records for southeastern Indonesia. The composite record shows pronounced shifts in monsoon rainfall that are antiphased with precipitation records for East Asia and the central-eastern equatorial Pacific. These meridional and zonal patterns are best explained by a poleward expansion of the Australasian Intertropical Convergence Zone and weakening of the Pacific Walker circulation (PWC) between similar to 1000 and 1500 CE Conversely, an equatorward contraction of the Intertropical Convergence Zone and strengthened PWC occurred between similar to 1500 and 1900 CE. Our findings, together with climate model simulations, highlight the likelihood that century-scale variations in tropical Pacific climate modes can significantly modulate radiatively forced shifts in global temperature

Tropical vegetation productivity and atmospheric methane over the last 40,000 years from model simulations and stalagmites in Sulawesi, Indonesia

Recent research has shown the potential of speleothem δ13C to record a range of environmental processes. Here, we report on 230Th-dated stalagmite δ13C records for southwest Sulawesi, Indonesia, over the last 40,000 yr to investigate the relationship between tropical vegetation productivity and atmospheric methane concentrations. We demonstrate that the Sulawesi stalagmite δ13C record is driven by changes in vegetation productivity and soil respiration and explore the link between soil respiration and tropical methane emissions using HadCM3 and the Sheffield Dynamic Global Vegetation Model. The model indicates that changes in soil respiration are primarily driven by changes in temperature and CO2, in line with our interpretation of stalagmite δ13C. In turn, modelled methane emissions are driven by soil respiration, providing a mechanism that links methane to stalagmite δ13C. This relationship is particularly strong during the last glaciation, indicating a key role for the tropics in controlling atmospheric methane when emissions from high-latitude boreal wetlands were suppressed. With further investigation, the link between δ13C in stalagmites and tropical methane could provide a low-latitude proxy complementary to polar ice core records to improve our understanding of the glacial–interglacial methane budget