ALBERT

Description: Author Posting. © American Geophysical Union, 2021. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography and Paleoclimatology 36(5), (2021): e2021PA004219, https://doi.org/10.1029/2021PA004219.

Description: The Yucatán Peninsula (YP) has a complex hydroclimate with many proposed drivers of interannual and longer-term variability, ranging from coupled ocean–atmosphere processes to frequency of tropical cyclones. The mid-Holocene, a time of higher Northern Hemisphere summer insolation, provides an opportunity to test the relationship between YP precipitation and ocean temperature. Here, we present a new, ∼annually resolved speleothem record of stable isotope (δ18O and δ13C) and trace element (Mg/Ca and Sr/Ca) ratios for a section of the mid-Holocene (5.2–5.7 kyr BP), before extensive agriculture began in the region. A meter-long stalagmite from Río Secreto, a cave system in Playa del Carmen, Mexico, was dated using U–Th geochronology and layer counting, yielding multidecadal age uncertainty (median 2SD of ±70 years). New proxy data were compared to an existing late Holocene stalagmite record from the same cave system, allowing us to examine changes in hydrology over time and to paleoclimate records from the southern YP. The δ18O, δ13C, and Mg/Ca data consistently indicate higher mean precipitation and lower precipitation variability during the mid-Holocene compared to the late Holocene. Despite this reduced variability, multidecadal precipitation variations were persistent in regional hydroclimate during the mid-Holocene. We therefore conclude that higher summer insolation led to increased mean precipitation and decreased precipitation variability in the northern YP but that the region is susceptible to dry periods across climate mean states. Given projected decreases in wet season precipitation in the YP’s near future, we suggest that climate mitigation strategies emphasize drought preparation.

Description: This work was funded by US National Science Foundation grants AGS-1702848 (M. Medina-Elizalde) and AGS-1502877 (S. Burns). This material is based on work supported by the National Science Foundation Graduate Research Fellowship under grant 1122374 (G. Serrato Marks). Additional support was provided by the MIT EAPS Student Research Fund and the WHOI Ocean Ventures Fund.

Description: 2021-11-06

Description: Author Posting. © The Author(s), 2014. This is the author's version of the work. It is posted here by permission of Elsevier for personal use, not for redistribution. The definitive version was published in Palaeogeography, Palaeoclimatology, Palaeoecology 415 (2014): 3-13, doi:10.1016/j.palaeo.2014.05.030.

Description: Reconstructions of surface paleoceanographic conditions of the western equatorial Atlantic and past climates of the adjacent Northeast Brazilian (the "Nordeste") continental margin were undertaken by analyzing sediments from a piston core and associated gravity and box cores recovered from 3107 meter water depth at 0° 20’ N on the equatorial Brazilian continental slope. The record is dated by radiocarbon analysis and oxygen isotopic stratigraphy of planktonic foraminifers and spans from near- modern to approximately 110 Ka. High-resolution XRF analysis provides insight into the paleoclimate history of the Nordeste during the last glacial interval. Several large-amplitude and abrupt peaks are observed in the time series of Ti/Ca and are usually accompanied by peaks of Fe/K. Together these record periods of increased precipitation and intense weathering on the adjacent continent and increased terrestrial sediment discharge from Nordeste rivers into the Atlantic. Within the limits of dating accuracy, most Ti/Ca peaks correlate with Heinrich events in the North Atlantic. This record thus corroborates, and extends back in time, the previous record of Arz et al (1998) determined on sediment cores from farther southeast along the Nordeste margin. Stable oxygen isotopic analysis and Mg/Ca paleothermometry on the near- surface-dwelling planktonic foraminiferal species Globierinoides ruber find that mean sea-surface temperature (SST) during glacial time (20 to 55 Ka, n = 97) was 23.89 ± 0.79 °C and the mean SST during the late Holocene (0 to 5 Ka, n = 14) was 26.89 ± 0.33 °C. SSTs were 0.5 to 2 °C higher and inferred sea-surface salinities were lower during most of the periods of elevated Ti/Ca, thus, as observed in previous studies, the western equatorial Atlantic was warm (at least locally) and the adjacent southern tropical continent was wet at the same time that the high-latitude North Atlantic was cold. Using the SYNTRACE-CCSM3 fully coupled climate model with transient forcing for the period 22 Ka to present, we find that decreased transport of the North Brazil Current co-occurs with reduced Atlantic meridional overturning circulation, and colder-than-normal SSTs in the North Atlantic region. These simulated conditions are invariably associated with significantly increased precipitation in the Nordeste region.

Description: Funding for the cruise and post-cruise science was provided to PAB by NSF-OCE-0823650.

Description: Ostracode species assemblages and stable oxygen and carbon isotope ratios of living and recent ostracodes, together with delta18O and delta13C_DIC values of host water samples, provide a first data set that characterizes a wide range of modern aquatic environments in the Laguna Cari-Laufquen (41°S, 68 - 69°W) and the Lago Cardiel area (48 - 49°S, 70 - 71°W) in Patagonia, Argentina. This data set will ultimately be used to interpret and calibrate data acquired from lake sediment cores with the goal of reconstructing past climate. Species assemblages and isotope values can be assigned to three groups; (1) springs, seeps and streams, (2) permanent ponds and lakes, and (3) ephemeral ponds and lakes. Springs, seeps and streams are characterized by Darwinula sp., Heterocypris incongruens, Eucypris fontana, Amphicypris nobilis and Ilyocypris ramirezi. Ostracode and water isotope values range between –13 and –5 per mil for oxygen, and between –15 and –3 per mil for carbon. They are the most negative of the entire sample set, reflecting ground water input with little or no evaporative enrichment. Limnocythere patagonica, Eucypris labyrinthica, Limnocythere sp. and Eucypris aff. fontana are typical species of permanent ponds and lakes. Isotope values indicate high degree of evaporation of lake waters relative to feeder springs and streams and range between –7 and +5 per mil for oxygen, and –5 and +4 per mil for carbon. Limnocythere rionegroensis is the dominant species in ephemeral ponds and lakes. These systems display the most enriched isotope values in both ostracodes and host waters, extending from –5 to +7 per mil for oxygen, and from –5 to +6 per mil for carbon. Living ostracodes show a positive offset from equilibrium values of up to 2 per mil for oxygen. Carbon-isotope values are up to 6‰ more negative than equilibrium values in highly productive pools. Comparison of ostracode and host water isotope signals permits assessment of the life span of the aquatic environments. Valves from dead ostracodes collected from ephemeral ponds and lakes show a wide scatter with each sample providing a snapshot of the seasonal history of the host water. The presence of the stream species Ilyocypris ramirezi and a wide range of ostracode isotope values suggest that ephemeral ponds and lakes are fed by streams during spring run-off and seasonally dry. A temporary character is also indicated by Heterocypris incongruens, a drought-resistant species that occupies most springs and seeps. In addition, Limnocythere rionegroensis has adjusted its reproduction strategies to its environment. Whereas only females were collected in fresh host waters, males were found in ephemeral ponds and lakes with higher solute content. Sexual reproduction seems to be the more successful reproduction strategy in high and variable salinities and seasonal droughts. The temporary character of the aquatic environments shows that the availability of meteoric water controls the life span of host waters and underlines the sensitivity of the area to changes in precipitation.

Description: A multiproxy approach including the use of stable isotopes, magnetic characterization analyses, and organic geochemistry has been adopted to consider factors such as productivity and terrigenous input over the past 1.5 m.y. at two areas off the western coast of Africa. These factors can, in turn, be used to consider variability in ocean circulation and upwelling in addition to changes in climate on the African continent. In particular, studies focused on the influence of glacial-interglacial cycles and evidence for the mid-Pleistocene revolution (MPR), a complex change in climate that occurred at ~1 Ma. A comparison of the records from the two areas drilled during Ocean Drilling Program Leg 175, the Congo Basin, at a latitude of 5°S (Holes 1076A and 1077A), and the Walvis Ridge, at 17°S (Hole 1081A), demonstrates that these sites are affected by different localized factors. The sites in the Congo Basin are strongly influenced by freshwater and sediment from the Congo River, whereas the site at the Walvis Ridge is located in the center of oceanic upwelling and contains a more marine signal. Evidence also suggests that the two sites responded differently to both long- and short-term climatic variations. In particular, the response at the Walvis Ridge to the MPR occurred over an extended period, from 1.1 to 0.8 Ma, and was associated with a change in the dominant source of terrigenous input to the site in conjunction with a change in the productivity signal. In the Congo Basin, the response to the MPR was more rapid, occurring between 0.9 and 0.8 Ma. During this period, the influence of the Congo River became significant. However, productivity records only began to respond toward the end of this interval, at 0.8 Ma.