Optimizing sampling strategies in high-resolution paleoclimate records

Abstract

Abstract. The aim of paleoclimate studies to resolve climate variability from noisy proxy records can in essence be reduced to a statistical problem. The challenge is to isolate meaningful information on climate events from these records by reducing measurement uncertainty through a combination of proxy data while retaining the temporal resolution needed to assess the timing and duration of the event. In this study, we explore the limits of this compromise by testing different methods for combining proxy data (smoothing, binning and sample size optimization) on a particularly challenging paleoclimate problem: resolving seasonal variability in stable isotope records. We test and evaluate the effects of changes in the seasonal temperature and hydrology cycle as well as changes in accretion rate of the archive and parameters such as sampling resolution and age model uncertainty on the reliability of seasonality reconstructions based on clumped and oxygen isotope analyses in 33 real and virtual datasets. Our results show that strategic combinations of clumped isotope analyses can significantly improve the accuracy of seasonality reconstructions if compared with conventional stable oxygen isotope analyses, especially in settings where the isotopic composition of the water is poorly constrained. Smoothing data using a moving average often leads to a dampening of the seasonal cycle, significantly reducing the accuracy of reconstructions. A statistical sample size optimization protocol yields more precise results than smoothing. However, the most accurate results are obtained through monthly binning of proxy data, especially in cases where growth rate or water composition cycles dampen the seasonal temperature cycle. Our analysis of a wide range of natural situations reveals that the effect of temperature seasonality on isotope records almost invariably exceeds that of changes in water composition. Thus, in most cases, isotope records allow reliable identification of growth seasonality as a basis for age modelling and seasonality reconstructions in absence of independent chronological markers in the record. These specific findings allow us to formulate general recommendations for sampling and combining data in paleoclimate research and have implications beyond the reconstruction of seasonality. We discuss the implications of our results for solving common problems in paleoclimatology and stratigraphy, including cyclostratigraphy, strontium isotope dating and event stratigraphy.

First absolute seasonal temperature estimates for greenhouse climate from clumped isotopes in bivalve shells

“…All specimens showed clear seasonal δ 18 Oc fluctuations of −2.0-0.0‰ in R. diluvianum, -2.0-0.0‰ in A. incurva and -2.7‰--1.0‰ in B. suecicus on which shell chronologies were based (see Methods). The assumption that periodic δ 18 Oc fluctuations reflect seasonality is demonstrated to be a valid basis for constructing intra-shell chronologies in nearly all modern environments 20 . Seasonal δ 18 Oc patterns show that the specimens record 3 (A. incurva and B. suecicus) to 6 (R. diluvianum) full years of growth.…”

“…The advent of carbonate clumped isotope (Δ47) SST reconstructions on a seasonal scale promises to eliminate these two biases 20 . The clumped isotope thermometer yields accurate SST reconstructions independent of δ 18 Osw assumptions 21,22 .…”

“…foraminifera, bivalves and eggshells) conform to the theoretical Δ47 temperature calibration 23,24 (see Supplementary Methods and Discussion). The large sample sizes required for individual Δ47-based temperature estimates (>2 mg) have complicated paleoseasonality reconstructions using this accurate method 25 , but a recently developed statistical approach enables its use for seasonality reconstructions 20 .…”

First absolute seasonal temperature estimates for greenhouse climate from clumped isotopes in bivalve shells

“…SST's are calculated from Δ47 values in monthly time bins (1/12 th of the seasonal cycle) using the temperature calibration by 51 recalculated in 23 , and δ 18 Osw is reconstructed from Δ47-SST and δ 18 Oc following 17 ( Supplementary Methods and Supplementary Data 3). The accuracy of this statistical approach for combining Δ47 aliquots for seasonal SST and δ 18 Osw reconstructions is tested on a diverse group of modern datasets and evaluated in 20 . It is demonstrated that this method achieves the ideal compromise between eliminating bias and retaining high reproducibility while keeping SST and δ 18 Osw reconstructions independent of the δ 18 Oc values on which the age model is based 20 (see also Supplementary Methods).…”

“…The advent of carbonate clumped isotope (Δ47) SST reconstructions on a seasonal scale promises to eliminate these two biases 20 . The clumped isotope thermometer yields accurate SST reconstructions independent of δ 18 Osw assumptions 21,22 .…”