Showing papers by "Gerald H. Haug published in 2023"

Disentangling multiproxy temperature reconstructions from the subtropical North Atlantic

Abstract: Reliable reconstruction of past sea surface temperature (SST) is of prime importance for understanding the Earth’s sensitivity to external forcing. Yet, it remains a major challenge in paleoceanography because comparison between SST estimates from different proxies reveals mismatches and raise the question as to what the contrasting proxies actually record. A better understanding of these mismatches in the light of seasonal occurrence of the proxy bearing organisms (archives) and water mass changes help to assess climate models. Here, we analyze data from the last deglaciation using a sediment core site situated at the northern boundary of the North Atlantic subtropical gyre influenced by fast latitudinal migrations of the subtropical Azores Front (AF) and resulting changes in water masses that may affect the SST records. Differences between the SST estimates from different deglacial SST reconstructions obtained from (1) Mg/Ca in planktic foraminifer tests, (2) alkenone UK′37, and (3) planktic foraminifer assemblages (SIMMAX), are assumed to result from the ecology of the proxy bearing organisms, and are assessed for the impact on different SST reconstructions from local seawater δ18O (δ18Ow) reconstructions. The general trends of SSTs from all four proxies confirm the well-known deglacial succession of warm and cold events. Mismatches between amplitudes of temperature changes are explained by differences in the phenology of the proxy-bearing organisms and local changes in hydrography. The combination of δ18O SST from the three different archives of δ18Ow reconstructions may cause offsets that exceed the climate driven signals.

Recurring summer and winter droughts from 4.2-3.97 thousand years ago in north India

Abstract: Abstract The 4.2-kiloyear event has been described as a global megadrought that transformed multiple Bronze Age complex societies, including the Indus Civilization, located in a sensitive transition zone with a bimodal (summer and winter) rainfall regime. Here we reconstruct changes in summer and winter rainfall from trace elements and oxygen, carbon, and calcium isotopes of a speleothem from Dharamjali Cave in the Himalaya spanning 4.2–3.1 thousand years ago. We find a 230-year period of increased summer and winter drought frequency between 4.2 and 3.97 thousand years ago, with multi-decadal aridity events centered on 4.19, 4.11, and 4.02 thousand years ago. The sub-annually resolved record puts seasonal variability on a human decision-making timescale, and shows that repeated intensely dry periods spanned multiple generations. The record highlights the deficits in winter and summer rainfall during the urban phase of the Indus Civilization, which prompted adaptation through flexible, self-reliant, and drought-resistant agricultural strategies.

The agulhas current transports signals of local and remote indian ocean nitrogen cycling

Abstract: The greater Agulhas Current region is an important component of the climate system, yet its influence on carbon and nutrient cycling is poorly understood. Here, we use nitrate isotopes (δ15N, δ18O, Δ(15–18) = δ15N–δ18O) to trace regional water mass circulation and investigate nitrogen cycling in the Agulhas Current and adjacent recirculating waters. The deep and intermediate waters record processes occurring remotely, including partial nitrate assimilation in the Southern Ocean and denitrification in the Arabian Sea. In the thermocline and surface, tropically sourced waters are biogeochemically distinct from adjacent subtropically sourced waters, confirming inhibited lateral mixing across the current core. (Sub)tropical thermocline nitrate δ15N is lower (4.9–5.8‰) than the sub-thermocline source, Subantarctic Mode Water (6.9‰); we attribute this difference to local N2 fixation. Using a one-box model to simulate the newly fixed nitrate flux, we estimate a local N2 fixation rate of 7–25 Tg N.a−1, with the upper limit likely biased high. In the mixed layer, nitrate δ15N and δ18O rise in unison, indicating that phytoplankton nitrate assimilation dominates in surface waters, with nitrification restricted to deeper waters. Because nitrate assimilation and nitrification are vertically decoupled, the rate of nitrate assimilation plus N2 fixation can be used to approximate carbon export. Thermocline and mixed-layer nitrate Δ(15–18) is low, due to both N2 fixation and coupled partial nitrate assimilation and nitrification. Similarly low-Δ(15–18) nitrate in Agulhas rings indicates leakage of low-δ15N nitrogen into the South Atlantic, which should be recorded in the organic matter sinking to the seafloor, providing a potential tracer of past Agulhas leakage.

Correction for Farmer et al., The Bering Strait was flooded 10,000 years before the Last Glacial Maximum

Abstract: Correction for “The Bering Strait was flooded 10,000 years before the Last Glacial Maximum,” by Jesse R. Farmer, Tamara Pico, Ona M. Underwood, Rebecca Cleveland Stout, Julie Granger, Thomas M. Cronin, François Fripiat, Alfredo Martínez-García, Gerald H. Haug, and Daniel M. Sigman, which published December 27, 2022; 10.1073/pnas.2206742119 (Proc. Natl. Acad. Sci. U.S.A. 120, e2206742119). The authors note that due to a production error, the vertical axis in Fig. 3B had been slightly offset from its correct position. The corrected figure and its legend appear below. Additionally, ref. 50 appeared incorrectly. The correct reference is M. L. Bender, Orbital tuning chronology for the Vostok climate record supported by trapped gas composition. Earth Planet. Sci. Lett. 204, 275–289 (2002). The online version has been corrected.