Laura Rodríguez-Sanz

Bipolar seesaw control on last interglacial sea level

Our current understanding of ocean–atmosphere–cryosphere interactions at ice-age terminations relies largely on assessments of the most recent (last) glacial–interglacial transition, Termination I (T-I). But the extent to which T-I is representative of previous terminations remains unclear. Testing the consistency of termination processes requires comparison of time series of critical climate parameters with detailed absolute and relative age control. However, such age control has been lacking for even the penultimate glacial termination (T-II), which culminated in a sea-level highstand during the last interglacial period that was several metres above present. Here we show that Heinrich Stadial 11 (HS11), a prominent North Atlantic cold episode, occurred between 135 ± 1 and 130 ± 2 thousand years ago and was linked with rapid sea-level rise during T-II. Our conclusions are based on new and existing data for T-II and the last interglacial that we collate onto a single, radiometrically constrained chronology. The HS11 cold episode punctuated T-II and coincided directly with a major deglacial meltwater pulse, which predominantly entered the North Atlantic Ocean and accounted for about 70 per cent of the glacial–interglacial sea-level rise. We conclude that, possibly in response to stronger insolation and CO2 forcing earlier in T-II, the relationship between climate and ice-volume changes differed fundamentally from that of T-I. In T-I, the major sea-level rise clearly post-dates Heinrich Stadial 1. We also find that HS11 coincided with sustained Antarctic warming, probably through a bipolar seesaw temperature response, and propose that this heat gain at high southern latitudes promoted Antarctic ice-sheet melting that fuelled the last interglacial sea-level peak.

Remanence acquisition efficiency in biogenic and detrital magnetite and recording of geomagnetic paleointensity

Relative paleointensity (RPI) variations of Earths magnetic field are widely used to understand geomagnetic field behavior and to develop age models for sedimentary sequences. RPI estimation is based on a series of assumptions. One key assumption that is rarely considered is that all magnetic particles in the sediment acquired a magnetization in an identical manner. In this paper, we test this assumption for sediments from the eastern equatorial Pacific Ocean that record well-documented global RPI variations over the last ∼780 kyr. The magnetization is carried by two stable single domain magnetic components, which we identify as magnetite magnetofossils and titanomagnetite nanoparticle inclusions within larger silicate particles. By analyzing signals carried by the two components separately, we determine for the first time that magnetic nanoparticle inclusions can cause their host particles to record reliable but inefficient sedimentary paleomagnetic signals. The magnetization carried by biogenic magnetite is acquired more efficiently than that carried by the nanoparticle inclusions. Variations in the concentration of both components are modulated climatically so that they record nearly identical RPI signals. In many sediment types, there is no correlation between the concentrations of different magnetic components so that variable remanence acquisition efficiency will complicate RPI recording. Our work demonstrates that detailed assessment of paleomagnetic recording by each constituent magnetic component needs to become a routine part of sedimentary RPI analysis.

A Reassessment of the Precision of Carbonate Clumped Isotope Measurements: Implications for Calibrations and Paleoclimate Reconstructions

Carbonate clumped isotopes offer a potentially transformational tool to interpret Earths history, but the proxy is still limited by poor interlaboratory reproducibility. Here, we focus on the uncertainties that result from the analysis of only a few replicate measurements to understand the extent to which unconstrained errors affect calibration relationships and paleoclimate reconstructions. We find that highly precise data can be routinely obtained with multiple replicate analyses, but this is not always done in many laboratories. For instance, using published estimates of external reproducibilities we find that typical clumped isotope measurements (three replicate analyses) have margins of error at the 95% confidence level (CL) that are too large for many applications. These errors, however, can be systematically reduced with more replicate measurements. Second, using a Monte Carlo-type simulation we demonstrate that the degree of disagreement on published calibration slopes is about what we should expect considering the precision of Δ47 data, the number of samples and replicate analyses, and the temperature range covered in published calibrations. Finally, we show that the way errors are typically reported in clumped isotope data can be problematic and lead to the impression that data are more precise than warranted. We recommend that uncertainties in Δ47 data should no longer be reported as the standard error of a few replicate measurements. Instead, uncertainties should be reported as margins of error at a specified confidence level (e.g., 68% or 95% CL). These error bars are a more realistic indication of the reliability of a measurement.

Mediterranean circulation perturbations over the last five centuries: Relevance to past Eastern Mediterranean Transient-type events

The Eastern Mediterranean Transient (EMT) occurred in the Aegean Sea from 1988 to 1995 and is the most significant intermediate-to-deep Mediterranean overturning perturbation reported by instrumental records. The EMT was likely caused by accumulation of high salinity waters in the Levantine and enhanced heat loss in the Aegean Sea, coupled with surface water freshening in the Sicily Channel. It is still unknown whether similar transients occurred in the past and, if so, what their forcing processes were. In this study, sediments from the Sicily Channel document surface water freshening (SCFR) at 1910 ± 12, 1812 ± 18, 1725 ± 25 and 1580 ± 30 CE. A regional ocean hindcast links SCFR to enhanced deep-water production and in turn to strengthened Mediterranean thermohaline circulation. Independent evidence collected in the Aegean Sea supports this reconstruction, showing that enhanced bottom water ventilation in the Eastern Mediterranean was associated with each SCFR event. Comparison between the records and multi-decadal atmospheric circulation patterns and climatic external forcings indicates that Mediterranean circulation destabilisation occurs during positive North Atlantic Oscillation (NAO) and negative Atlantic Multidecadal Oscillation (AMO) phases, reduced solar activity and strong tropical volcanic eruptions. They may have recurrently produced favourable deep-water formation conditions, both increasing salinity and reducing temperature on multi-decadal time scales.

Penultimate deglacial warming across the Mediterranean Sea revealed by clumped isotopes in foraminifera

The variability of seawater temperature through time is a critical measure of climate change, yet its reconstruction remains problematic in many regions. Mg/Ca and oxygen isotope (δ18OC) measurements in foraminiferal carbonate shells can be combined to reconstruct seawater temperature and δ18O (δ18OSW). The latter is a measure of changes in local hydrology (e.g., precipitation/evaporation, freshwater inputs) and global ice volume. But diagenetic processes may affect foraminiferal Mg/Ca. This restricts its potential in many places, including the Mediterranean Sea, a strategic region for deciphering global climate and sea-level changes. High alkalinity/salinity conditions especially bias Mg/Ca temperatures in the eastern Mediterranean (eMed). Here we advance the understanding of both western Mediterranean (wMed) and eMed hydrographic variability through the penultimate glacial termination (TII) and last interglacial, by applying the clumped isotope (Δ47) paleothermometer to planktic foraminifera with a novel data-processing approach. Results suggest that North Atlantic cooling during Heinrich stadial 11 (HS11) affected surface-water temperatures much more in the wMed (during winter/spring) than in the eMed (during summer). The method’s paired Δ47 and δ18OC data also portray δ18OSW. These records reveal a clear HS11 freshwater signal, which attenuated toward the eMed, and also that last interglacial surface warming in the eMed was strongly amplified by water-column stratification during the deposition of the organic-rich (sapropel) interval known as S5.

Corrigendum: Bipolar seesaw control on last interglacial sea level.

This corrects the article DOI: 10.1038/nature14499