Little Ice Age abruptly triggered by intrusion of Atlantic waters into the Nordic Seas

The Little Ice Age (LIA) was one of the coldest periods of the postglacial period in the Northern Hemisphere. Although there is increasing evidence that this time interval was associated with weakening of the subpolar gyre (SPG), the sequence of events that led to its weakened state has yet to be explained. Here, we show that the LIA was preceded by an exceptional intrusion of warm Atlantic water into the Nordic Seas in the late 1300s. The intrusion was a consequence of persistent atmospheric blocking over the North Atlantic, linked to unusually high solar activity. The warmer water led to the breakup of sea ice and calving of tidewater glaciers; weakening of the blocking anomaly in the late 1300s allowed the large volume of ice that had accumulated to be exported into the North Atlantic. This led to a weakening of the SPG, setting the stage for the subsequent LIA

GDGT Distribution in a Stratified Lake and Implications for the Application of TEX86 in Paleoenvironmental Reconstructions

We investigated the relationship between distributions of GDGTs, GDGT-based proxies and environmental factors in a stratified lake in northwestern Norway. More than 90% of isoGDGTs were produced at the bottom of the oxycline, indicating a predominance of ammonia-oxidizing Group I.1a of Thaumarchaeota, supported by high crenarchaeol/caldarchaeol ratios. Dissolved oxygen content, rather than temperature, exercised a primary control on TEX86 values. In spite of low BIT value in surface sediment, the reconstructed lake surface temperature was “cold” biased. MBT values in streams and lake surface water were significantly smaller than those in the catchment soil, suggesting in situ production of brGDGTs in streams. A rapid transition of MBT vs. temperature/pH relationships occurring at the bottom of oxycline indicated the differential production of various brGDGTs with D.O. and depths. Only within the oxycline were CBT-based pH values close to in situ pH. Our results confirm earlier studies calling for caution in applying TEX86 as a surface temperature proxy, or MBT and/or CBT for reconstructing pH, in anoxic or euxinic lakes, estuaries and ocean basins. We propose that caldarchaeol/crenarchaeol ratio, an indicator of contributions from methanogenic archaea, together with the BIT and TEX86 proxies, can help reconstruct past levels of stratification

Arctic sea ice export as a driver of deglacial climate

© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Condron, A., Joyce, A. J., & Bradley, R. S. Arctic sea ice export as a driver of deglacial climate. Geology, 48(4), (2020): 395-399, doi:10.1130/G47016.1.A widespread theory in paleoclimatology suggests that changes in freshwater discharge to the Nordic (Greenland, Norwegian, and Icelandic) Seas from ice sheets and proglacial lakes over North America played a role in triggering episodes of abrupt climate change during deglaciation (21–8 ka) by slowing the strength of the Atlantic Meridional Overturning circulation (AMOC). Yet, proving this link has been problematic, as climate models are unable to produce centennial-to-millennial–length reductions in overturning from short-lived outburst floods, while periods of iceberg discharge during Heinrich Event 1 (ca. 16 ka) may have occurred after the climate had already begun to cool. Here, results from a series of numerical model experiments are presented to show that prior to deglaciation, sea ice could have become tens of meters thick over large parts of the Arctic Basin, forming an enormous reservoir of freshwater independent from terrestrial sources. Our model then shows that deglacial sea-level rise, changes in atmospheric circulation, and terrestrial outburst floods caused this ice to be exported through Fram Strait, where its subsequent melt freshened the Nordic Seas enough to weaken the AMOC. Given that both the volume of ice stored in the Arctic Basin and the magnitude of the simulated export events exceed estimates of the volumes and fluxes of meltwater periodically discharged from proglacial Lake Agassiz, our results show that non-terrestrial freshwater sources played an important role in causing past abrupt climate change.The research was supported by the National Science Foundation (NSF) through NSF grants ARC-1204045 and PLR-1417667. The numerical simulations were carried out using MITgcm (http://mitgcm.org) and used resources provided by the Extreme Science and Engineering Discovery Environment (XSEDE; https://www.xsede.org), which is supported by NSF grant number ACI-1053575

Proxy-to-proxy calibration: Increasing the temporal resolution of quantitative climate reconstructions

High-resolution paleoclimate reconstructions are often restricted by the difficulties of sampling geologic archives in great detail and the analytical costs of processing large numbers of samples. Using sediments from Lake Braya Sø, Greenland, we introduce a new method that provides a quantitative high-resolution paleoclimate record by combining measurements of the alkenone unsaturation index ([Image: see text]) with non-destructive scanning reflectance spectroscopic measurements in the visible range (VIS-RS). The proxy-to-proxy (PTP) method exploits two distinct calibrations: the in situ calibration of [Image: see text] to lake water temperature and the calibration of scanning VIS-RS data to down core [Image: see text] data. Using this approach, we produced a quantitative temperature record that is longer and has 5 times higher sampling resolution than the original [Image: see text] time series, thereby allowing detection of temperature variability in frequency bands characteristic of the AMO over the past 7,000 years

Anomalías de la precipitación invernal en la Península Ibérica y Baleares después de grandes erupciones volcánicas tropicales (1901-1996)

Ponencia presentada en: III Congreso de la Asociación Española de Climatología “El agua y el clima”, celebrado en Palma de Mallorca del 16 al 19 de junio de 2002.[ES]En este artículo se analizan las anomalías de precipitación sobre la Península Ibérica y Baleares durante los inviernos posteriores a grandes erupciones volcánicas tropicales del siglo XX. Para ello, se utiliza el método del Análisis de Épocas Superpuestas para resaltar la señal volcánica del ruido de fondo. Los resultados obtenidos muestran significativas anomalías negativas de precipitación en el extremo noroeste de la Península, durante los dos primeros inviernos posteriores a grandes eventos volcánicos, mientras que anomalías de signo contrario son perceptibles en el área próxima al Mediterráneo para el mismo período. Estos resultados parecen estar en consonancia con anomalías de circulación sobre las latitudes medias previamente detectadas, constatándose un incremento de los vientos del oeste alrededor del paralelo 60ºN.[EN]Winter precipitation anomalies over the Iberian Peninsula and the Balearic Islands following major tropical volcanic eruptions of the 20th century are discussed. A Superposed Epoch Analysis method is applied in order to identify the volcanic signal from de background noise. The results obtained show significant negative rainfall anomalies in the north-western sector of Iberia within the first two winters after large eruptions, while positive anomalies are detected in the area close to the Mediterranean Sea. These results are in agreement with circulation anomalies over the mid-latitudes previously reported, in the sense of increased tropospheric westerlies at 60ºN.Esta comunicación se ha llevado a cabo gracias al proyecto REN2001-2865-C02-01 (Ministerio de Ciencia y Tecnología) y en el marco de las actividades del Grup de Climatologia (Generalitat de Catalunya)

Future Decreases in Freezing Days across North America

This study used air temperatures from a suite of regional climate models participating in the North American Climate Change Assessment Program (NARCCAP) together with two atmospheric reanalysis datasets to investigate changes in freezing days (defined as days with daily average temperature below freezing) likely to occur between 30-yr baseline (1971–2000) and midcentury (2041–70) periods across most of North America. Changes in NARCCAP ensemble mean winter temperature show a strong gradient with latitude, with warming of over 4°C near Hudson Bay. The decline in freezing days ranges from less than 10 days across north-central Canada to nearly 90 days in the warmest areas of the continent that currently undergo seasonally freezing conditions. The area experiencing freezing days contracts by 0.9–1.0 × 106 km2 (5.7%–6.4% of the total area). Areas with mean annual temperature between 2° and 6°C and a relatively low rate of change in climatological daily temperatures (−) near the time of spring thaw will encounter the greatest decreases in freezing days. Advances in the timing of spring thaw will exceed the delay in fall freeze across much of the United States, with the reverse pattern likely over most of Canada

Diatom and stable isotope records of late-Holocene lake ontogeny at Indrepollen, Lofoten, NW Norway: a response to glacio-isostasy and Neoglacial cooling

Borg Indrepollen is a coastal lake on the island of Vestvågøy, Lofoten, NW Norway. A sedimentary sequence spanning the last 4500 cal. yr BP was analysed for diatom, C/N and stable isotopes (δ13C and δ15 N). The record provides clear evidence of glacio-isostatic rebound in the Lofoten region. Five distinct lithostratigraphic-geochemical zones (I—V) and four major diatom zones (A—D) were identified. The early record (I—III, A—Biii; 4500—550 cal. yr BP) contains coarse sedimentary material and diatoms indicative of more marine conditions. The correlation between the isotopic data (high δ13C and low C/N) and trends in the biological data (marine/brackish flora) suggest the marine influence on the Indrepollen basin is a controlling factor of the source of organic material. The latter part of the record (IV, C; 550 cal. yr 550—226) indicates a transitional phase from marine-dominated diatom to brackish taxa and is coincident with a section of microlaminations in the core. The youngest zone (V, D; 226 cal. yr BP—present) is indicative of an isolated basin, with the deposition of fine sediments; freshwater diatom taxa dominate the biological record and δ13C are indicative of freshwater conditions. C/N suggests a shift to a terrestrial source of organic matter. The proxies show a transition from full marine conditions, when Indrepollen was a submerged fjord, to more freshwater, lacustrine conditions in the last 200 years. The record of land uplift from Borg Indrepollen mirrors changes in sedimentary records from across Northern Norway and relative sea-level curves for the region