Search

High-resolution biogenic and geologic proxies in which one increment or layer is formed per year are crucial to describing natural ranges of environmental variability in Earth's physical and biological systems. However, dating controls are necessary to ensure temporal precision and accuracy; simple counts cannot ensure that all layers are placed correctly in time. Originally developed for tree-ring data, crossdating is the only such procedure that ensures all increments have been assigned the correct calendar year of formation. Here, we use growth-increment data from two tree species, two marine bivalve species, and a marine fish species to illustrate sensitivity of environmental signals to modest dating error rates. When falsely added or missed increments are induced at one and five percent rates, errors propagate back through time and eliminate high-frequency variability, climate signals, and evidence of extreme events while incorrectly dating and distorting major disturbances or other low-frequency processes. Our consecutive Monte Carlo experiments show that inaccuracies begin to accumulate in as little as two decades and can remove all but decadal-scale processes after as little as two centuries. Real-world scenarios may have even greater consequence in the absence of crossdating. Given this sensitivity to signal loss, the fundamental tenets of crossdating must be applied to fully resolve environmental signals, a point we underscore as the frontiers of growth-increment analysis continue to expand into tropical, freshwater, and marine environments., (© 2016 John Wiley & Sons Ltd.)

Assessing temporal variability in extreme rainfall events before the historical era is complicated by the sparsity of long-term "direct" storm proxies. Here we present a 2,200-y-long, accurate, and precisely dated record of cave flooding events from the northwest Australian tropics that we interpret, based on an integrated analysis of meteorological data and sediment layers within stalagmites, as representing a proxy for extreme rainfall events derived primarily from tropical cyclones (TCs) and secondarily from the regional summer monsoon. This time series reveals substantial multicentennial variability in extreme rainfall, with elevated occurrence rates characterizing the twentieth century, 850-1450 CE (Common Era), and 50-400 CE; reduced activity marks 1450-1650 CE and 500-850 CE. These trends are similar to reconstructed numbers of TCs in the North Atlantic and Caribbean basins, and they form temporal and spatial patterns best explained by secular changes in the dominant mode of the El Niño/Southern Oscillation (ENSO), the primary driver of modern TC variability. We thus attribute long-term shifts in cyclogenesis in both the central Australian and North Atlantic sectors over the past two millennia to entrenched El Niño or La Niña states of the tropical Pacific. The influence of ENSO on monsoon precipitation in this region of northwest Australia is muted, but ENSO-driven changes to the monsoon may have complemented changes to TC activity.

Rationale: Research on the ocean carbon cycle is vitally important due to the projected impacts of atmospheric CO2 on global temperatures and climate change, but also on ocean chemistry. The direct influence of this CO2 rise on the seawater pH can be evaluated from the boron isotopic composition in biogenic carbonates; however, conscientious laboratory techniques and data treatment are vital in obtaining accurate and precise results. A rapid-throughput boron purification and Total Evaporation Thermal Ionization Mass Spectrometry method was developed for high accuracy and precision boron isotopic analysis for small (ng) sample sizes., Methods: An improved microsublimation method, in which up to 20 samples can be processed simultaneously under identical temperature conditions, was developed. Several tests have confirmed the viability of this technique. First, seawater and Porites coral samples were processed with H2 O2 and the results compared with those obtained using microsublimation; second, the impact of various sublimation times was evaluated; and third, quantitative recovery was assessed using standard addition., Results: Microsublimation provides a valid method for the quantitative recovery and separation of boron from both major elements and organic matter under low-blank conditions. The close agreement of our results with published values validates the accuracy of the measurements. The isotopic ratio for SRM 951a boric acid isotopic standard was 4.0328 ± 0.0054 (2 STD, n = 25). The reproducibility of boron isotopic composition for standards including AE121, IAEA B-1 and an in-house coral standard UM-CP1 was ±0.68‰ (2 STD, n = 15), ±1.12‰ (2 STD, n = 24), and ±1.17‰ (2 STD, n = 14), respectively. The sample sizes were <1 ng for each measurement., Conclusions: The developed method for preparing and measuring boron isotopic values in a variety of carbonate materials should facilitate the reconstruction of past ocean pH conditions with decadal-scale resolution., (Copyright © 2013 John Wiley & Sons, Ltd.)

Despite numerous investigations, the dynamical origins of the Medieval Climate Anomaly and the Little Ice Age remain uncertain. A major unresolved issue relating to internal climate dynamics is the mode and tempo of Atlantic meridional overturning circulation variability, and the significance of decadal-to-centennial scale changes in Atlantic meridional overturning circulation strength in regulating the climate of the last millennium. Here we use the time-constrained high-resolution local radiocarbon reservoir age offset derived from an absolutely dated annually resolved shell chronology spanning the past 1,350 years, to reconstruct changes in surface ocean circulation and climate. The water mass tracer data presented here from the North Icelandic shelf, combined with previously published data from the Arctic and subtropical Atlantic, show that surface Atlantic meridional overturning circulation dynamics likely amplified the relatively warm conditions during the Medieval Climate Anomaly and the relatively cool conditions during the Little Ice Age within the North Atlantic sector.