Sebastian F.M. Breitenbach

Development and Disintegration of Maya Political Systems in Response to Climate Change

Maya and Climate Climate has affected the vitality of many different societies in the past, as shown by numerous records across the globe and throughout human history. One of the most obvious and spectacular examples of this is from the Classic Maya civilization, whose advanced culture left highly detailed records of all aspects of their existence between 300 and 1000 C.E. Kennett et al. (p. 788; see the cover) present a detailed climate record derived from a stalagmite collected from a cave in Belize, in the midst of the Classic Maya settlement. The fine resolution and precise dating of the record allows changes in precipitation to be related to the politics, war, and population fluctuations of the Mayans. A record of rainfall from a stalagmite in southern Belize provides a context for better understanding Maya civilization. The role of climate change in the development and demise of Classic Maya civilization (300 to 1000 C.E.) remains controversial because of the absence of well-dated climate and archaeological sequences. We present a precisely dated subannual climate record for the past 2000 years from Yok Balum Cave, Belize. From comparison of this record with historical events compiled from well-dated stone monuments, we propose that anomalously high rainfall favored unprecedented population expansion and the proliferation of political centers between 440 and 660 C.E. This was followed by a drying trend between 660 and 1000 C.E. that triggered the balkanization of polities, increased warfare, and the asynchronous disintegration of polities, followed by population collapse in the context of an extended drought between 1020 and 1100 C.E.

Trends and oscillations in the Indian summer monsoon rainfall over the last two millennia

Observations show that summer rainfall over large parts of South Asia has declined over the past five to six decades. It remains unclear, however, whether this trend is due to natural variability or increased anthropogenic aerosol loading over South Asia. Here we use stable oxygen isotopes in speleothems from northern India to reconstruct variations in Indian monsoon rainfall over the last two millennia. We find that within the long-term context of our record, the current drying trend is not outside the envelope of monsoons oscillatory variability, albeit at the lower edge of this variance. Furthermore, the magnitude of multi-decadal oscillatory variability in monsoon rainfall inferred from our proxy record is comparable to model estimates of anthropogenic-forced trends of mean monsoon rainfall in the 21st century under various emission scenarios. Our results suggest that anthropogenic-forced changes in monsoon rainfall will remain difficult to detect against a backdrop of large natural variability.

Long-term performance of the Kiel carbonate device with a new correction scheme for clumped isotope measurements

RATIONALE Carbonate clumped isotope thermometry is a powerful new technique increasingly used in many fields in earth science. Recently, it has been shown that clumped isotope measurements can be performed with a Kiel carbonate preparation device and micro-volume analyses, allowing measurements of small (1.5-2 mg) carbonate samples. However, common data correction schemes rely on measurements of gases prepared offline, potentially leading to unrecognized biases in the results. METHODS We propose a new correction scheme for the Kiel device method including: (1) A pressure-sensitive baseline correction (PBL) of the raw beam signals; (2) Transfer of data to the absolute reference frame; (3) Correction for acid fractionation; (4) Correction for average standard offsets; (5) When necessary, correction for Δ47 scale compression based on offsets among standards with different ordering state. The long-term performance of the new scheme was tested with a large set of standard measurements (N = 432) obtained over the course of 15 months. RESULTS The PBL correction reliably removes composition-dependent artifacts, which are commonly corrected for with gas measurements, and offsets observed in micro-volume measurements when ion beams are imbalanced. We show that the shape of the PBL can vary strongly and needs to be properly characterized. Combined PBL and standard correction resulted in long-term stability with standard deviations in Δ47 of 0.012-0.016 ‰ for the five standards over the whole period, close to the average error of 0.011 ‰ observed for individual measurements consisting of 10 replicate analyses. CONCLUSIONS Our correction scheme eliminates the need for routine gas measurements, allowing for equal treatment of samples and standards with the Kiel device setup. While the PBL and standard data obtained over 15 months reveal variable mass spectrometer behavior, they provide a robust means of correction, yielding reproducible results from small carbonate samples in the long term.

Late Holocene Asian summer monsoon dynamics from small but complex networks of paleoclimate data

Internal variability of the Asian monsoon system and the relationship amongst its sub-systems, the Indian and East Asian Summer Monsoon, are not sufficiently understood to predict its responses to a future warming climate. Past environmental variability is recorded in Palaeoclimate proxy data. In the Asian monsoon domain many records are available, e.g. from stalagmites, tree-rings or sediment cores. They have to be interpreted in the context of each other, but visual comparison is insufficient. Heterogeneous growth rates lead to uneven temporal sampling. Therefore, computing correlation values is difficult because standard methods require co-eval observation times, and sampling-dependent bias effects may occur. Climate networks are tools to extract system dynamics from observed time series, and to investigate Earth system dynamics in a spatio-temporal context. We establish paleoclimate networks to compare paleoclimate records within a spatially extended domain. Our approach is based on adapted linear and nonlinear association measures that are more efficient than interpolation-based measures in the presence of inter-sampling time variability. Based on this new method we investigate Asian Summer Monsoon dynamics for the late Holocene, focusing on the Medieval Warm Period (MWP), the Little Ice Age (LIA), and the recent period of warming in East Asia. We find a strong Indian Summer Monsoon (ISM) influence on the East Asian Summer Monsoon during the MWP. During the cold LIA, the ISM circulation was weaker and did not extend as far east. The most recent period of warming yields network results that could indicate a currently ongoing transition phase towards a stronger ISM penetration into China. We find that we could not have come to these conclusions using visual comparison of the data and conclude that paleoclimate networks have great potential to study the variability of climate subsystems in space and time.

Speleothems Reveal 500,000-year History of Siberian Permafrost

Permafrost Thaw Predictions Permafrost contains twice as much carbon as the atmosphere which could have serious consequences if it were to be released by widespread thawing. Vaks et al. (p. 183, published online 21 February) present a 450,000 year-long record of speleothem growth at selected locations in Siberia, which traces changes in the extent of permafrost over that time period. The authors conclude that conditions only slightly warmer than those of today would cause widespread thawing of continuous permafrost as far north as 60°N. Siberian caves recorded the history of permafrost occurrence during the past 450,000 years. Soils in permafrost regions contain twice as much carbon as the atmosphere, and permafrost has an important influence on the natural and built environment at high northern latitudes. The response of permafrost to warming climate is uncertain and occurs on time scales longer than those assessed by direct observation. We dated periods of speleothem growth in a north-south transect of caves in Siberia to reconstruct the history of permafrost in past climate states. Speleothem growth is restricted to full interglacial conditions in all studied caves. In the northernmost cave (at 60°N), no growth has occurred since Marine Isotopic Stage (MIS) 11. Growth at that time indicates that global climates only slightly warmer than today are sufficient to thaw extensive regions of permafrost.

Carbon and oxygen isotope analysis of small carbonate samples (20 to 100 µg) with a GasBench II preparation device

Dear Editor, Since the pioneering work by Emiliani and Shackleton, the carbon and oxygen stable isotopes of carbonates have been of major importance in geological and environmental sciences.[1-8] Palaeoceanographic and terrestrial climate research (e.g. on secondary cave carbonates or lacustrine sediments) routinely make use of the δ13C and δ18O of carbonates as climate proxies.[9-12] Online sample preparation and continuous-flow isotope ratio mass spectrometry (IRMS) with multi-loop injection procedures are now widely applied analytical techniques, because they allow precise and accurate measurements at high sample throughput.[13] The samples sizes used in analytical systems for carbonates described in the literature are generally larger than ca. 50 µg, with an external precision of ca. 0.05–0.10‰.[14, 15] Unfortunately, sample size is often a limiting factor in studies of many climate records (lacustrine sediments, speleothems, foraminifera) when sampling for individual sediment layers to obtain sub-annual to seasonal information.[16, 17] Reliable, accurate, and fast analysis of samples as small as possible is desired for ultra-high-resolution sampling for palaeoclimate investigations, but also for other geological applications, such as investigations of traces of carbonates in silicate rocks.[9, 18-20] Fiebig and co-workers described a method for the precise measurement of carbonate samples in the range 10 to 30 µg for the GasBench II, the same device as we describe in this communication.[21] However, their method requires a modification of the hardware and the use of liquid nitrogen, which increases the costs and the complexity of the measurements. In this letter we describe the improvements in analytical performance possible by simply reducing the size of sample vials from 12 to 4.5 mL and show that precise and accurate measurements on samples as small as 20 µg of carbonate are possible. For the experiments described below we employ the ThermoFinnigan (Bremen, Germany, now Thermo Fisher Scientific) GasBench II, equipped with a CTC autosampler (CTC Analytics AG, Zwingen, Switzerland), and coupled to a ConFlow IV interface and a Delta V Plus mass spectrometer (both Thermo Fisher Scientific) at the ETH Zurich (Zurich, Switzerland). The system setup is essentially the same as used in an earlier study.[13] Instead of the 12 mL exetainers normally used, we load micro-samples (7–150 µg) of carbonate into 4.5 mL round-bottomed borosilicate vials (Labco, High Wycombe, UK, part No. 948 W), capped with Labco butyl rubber septa.[13] Samples were weighed using a Mettler Toledo MT5 FACT microbalance (reproducibility 0.8 µg; Leicester, UK). The exetainers were flushed for 240 s on a model 222XL autosampler (Gilson™, Middleton, WI, USA), with helium grade 5.0 (99.999% He) at a flow rate of 55 mL/min, corresponding to a flushing volume of 220 mL. The smaller 4.5 mL exetainers allow the reduction of helium consumption compared with the 12 mL vials, as no air contamination was observed during our experiments (Fig. 1). Eight-one exetainers (including 18 in-house standards (powdered Carrara marble, MS2) are subsequently heated to 72 ± 0.1 °C in the aluminium block of the autosampler and acid-digested following standard procedures.[13] The heating period of the autosampler heating block is somewhat longer for short vials, due to the air in the heating block below the vials. If small vial runs are planned for an extended period of time, metal cylinders placed below the vials can be used to circumvent the problem.

Background effects on Faraday collectors in gas‐source mass spectrometry and implications for clumped isotope measurements

RATIONALE The measurement of the abundances of minor isotopologues by mass spectrometry requires correction of subtle non-linearities in the mass spectrometer that cause deviations in the relationship between actual and measured isotope ratios. Here we show that negative backgrounds on the Faraday cups recording the minor ion beams are the cause of the observed non-linearities in the measurement of CO(2) isotopologues, and propose a new correction procedure for clumped isotope measurements. METHODS We carefully investigated the cause of non-linearity effects in the measurement of the abundance of (13)C(18)O(16)O, a minor isotopologue of CO(2) with m/z 47, on two different mass spectrometers. By using gases of different composition with close to stochastic and with non-random distribution of isotopes we demonstrate that the apparent dependence of the excess abundance of the isotopologue of m/z 47 on the bulk isotopic composition of CO(2) is due to a background interference that is linearly dependent on the partial pressure of the gas in the source of the mass spectrometer. CONCLUSIONS Background determination with gas flowing into the source of the mass spectrometer is necessary for accurate clumped isotope measurements of CO(2). Background corrections can be performed accurately if the slit width of the m/z 44 Faraday cup significantly exceeds that of the one for m/z 47, using a correlation between m/z 44 signal intensity and the corresponding minimum in m/z 47 background. We propose two new correction schemes that reduce the time-consuming measurement of gases of different bulk isotopic compositions. These findings may also be relevant for the measurement of other rare isotopologues by mass spectrometry.

Indian monsoon variability on millennial-orbital timescales

The Indian summer monsoon (ISM) monsoon is critical to billions of people living in the region. Yet, significant debates remain on primary ISM drivers on millennial-orbital timescales. Here, we use speleothem oxygen isotope (δ18O) data from Bittoo cave, Northern India to reconstruct ISM variability over the past 280,000 years. We find strong coherence between North Indian and Chinese speleothem δ18O records from the East Asian monsoon domain, suggesting that both Asian monsoon subsystems exhibit a coupled response to changes in Northern Hemisphere summer insolation (NHSI) without significant temporal lags, supporting the view that the tropical-subtropical monsoon variability is driven directly by precession-induced changes in NHSI. Comparisons of the North Indian record with both Antarctic ice core and sea-surface temperature records from the southern Indian Ocean over the last glacial period do not suggest a dominant role of Southern Hemisphere climate processes in regulating the ISM variability on millennial-orbital timescales.

Seasonality of westerly moisture transport in the East Asian summer monsoon and its implications for interpreting precipitation δ18O

East Asian summer monsoon (EASM) rainfall impacts the worlds most populous regions. Accurate EASM rainfall prediction necessitates robust paleoclimate reconstructions from proxy data and quantitative linkage to modern climatic conditions. Many precisely dated oxygen isotope records from Chinese stalagmites have been interpreted as directly reflecting past EASM rainfall amount variability, but recent research suggests that such records instead integrate multiple hydroclimatic processes. Using a Lagrangian precipitation moisture source diagnostic, we demonstrate that EASM rainfall is primarily derived from the Indian Ocean. Conversely, Pacific Ocean moisture export peaks during winter, and the moisture uptake area does not differ significantly between summer and winter and is thus a minor contributor to monsoonal precipitation. Our results are substantiated by an accurate reproduction of summer and winter spatial rainfall distributions across China. We also correlate modern EASM rainfall oxygen isotope ratios with instrumental rainfall amount and our moisture source data. This analysis reveals that the strength of the source effect is geographically variable, and differences in atmospheric moisture transport may significantly impact the isotopic signature of EASM rainfall at the Hulu, Dongge, and Wanxiang Cave sites. These results improve our ability to isolate the rainfall amount signal in paleomonsoon reconstructions and indicate that precipitation across central and eastern China will directly respond to variability in Indian Ocean moisture supply.

Northeast Indian stalagmite records Pacific decadal climate change: Implications for moisture transport and drought in India

Two types of El Nino events are distinguished by sea surface temperature (SST) anomalies centered in the central or eastern equatorial Pacific. The Central Pacific El Nino events (CP-El Nino) are more highly correlated with weakening of the central Indian Summer Monsoon and linked to decadal Pacific climate variability. We present a 50 year, subannually resolved speleothem δ18O record from northeast India that exhibits a significant correlation with northern Pacific decadal variability and central equatorial Pacific SSTs. Accordingly, we suggest that δ18O time series in similar northeast Indian speleothems are effective tools for investigating preinstrumental changes in Pacific climate, including changes in El Nino dynamics. In contrast to central India, rainfall amounts in northeast India are relatively unaffected by El Nino. However, back trajectory analysis indicates that during CP-El Nino events moisture transport distance to northeast India is reduced, suggesting that variations in moisture transport primarily control δ18O in the region.