Pleistocene vertical carbon isotope and carbonate gradients in the South Atlantic sector of the Southern Ocean

We demonstrate that the carbon isotopic signal of mid-depth waters evolved differently from deep waters in the South Atlantic sector of the Southern Ocean during the Pleistocene. Deep sites (>3700 m) exhibit large glacial-to-interglacial variations in benthic d13C, whereas the amplitude of the d13C signal at Site 1088 (2100 m water depth) is small. Unlike the deep sites, at no time during the Pleistocene were benthic d13C values at Site 1088 lower than those of the deep Pacific. Reconstruction of intermediate-todeep d13C gradients (D13CI-D) supports the existence of a sharp chemocline between 2100 and 2700 m during most glacial stages of the last 1.1 myr. This chemical divide in the glacial Southern Ocean separated well-ventilated water above 2500 m from poorly ventilated water below. The D13CI-D signal parallels the Vostok atmospheric pCO2 record for the last 400 kyr, lending support to physical models that invoke changes in Southern Ocean deep water ventilation as a mechanism for changing atmospheric pCO2. The emergence of a strong 100-kyr cycle in D13CI-D during the mid-Pleistocene supports a change in vertical fractionation and deep-water ventilation rates in the Southern Ocean, and is consistent with possible CO2- forcing of this climate transition. Components: 7562 words, 14 figures, 2 tables

Atlantic Ocean thermohaline circulation changes on orbital to suborbital timescales during the mid-Pleistocene

Mid-Pleistocene benthic ∂18O and ∂13C time series from the North Atlantic site 983 and Ceara Rise site 928 are compared to an array of existing isotopic records spanning the Atlantic basin and the geographic extremes of the North Atlantic Deep Water/Southern Ocean Water interface during both glacial and interglacial periods. This comparison allows the persistent millennial-scale intermediate depth North Atlantic ventilation changes recorded at site 983 to be placed within the context of the longer period water mass reorganizations taking place throughout the mid-Pleistocene. Our benthic ∂13C results suggest that the intermediate depth North Atlantic experienced millennial-scale changes in ventilation throughout the mid-Pleistocene climate shift. The times of poorest ventilation (low benthic ∂13C) persisted for only a few millennia and were associated with rapid decreases in benthic ∂18O, suggesting that ice sheet decay and melt water induced salinity changes were effective at throttling deep water production in the North Atlantic throughout the mid-Pleistocene. Similar but less pronounced decreases in the ∂13C of the middepth waters also punctuated interglacials, suggesting that large ice sheet fluctuations do not explain all of the observed thermohaline circulation mode shifts in the North Atlantic. Meanwhile, on orbital timescales, glacial deep to intermediate water ∂13C gradients evolved after ~0.95 Ma. Taken together, these observations provide a number of new constraints for understanding the timing and evolution of deep water circulation changes across the mid-Pleistocene

North Atlantic Intermediate to Deep Water circulation and chemical stratification during the past 1 Myr

Benthic foraminiferal carbon isotope records from a suite of drill sites in the North Atlantic are used to trace variations in the relative strengths of Lower North Atlantic Deep Water (LNADW), Upper North Atlantic Deep Water (UNADW), and Southern Ocean Water (SOW) over the past 1 Myr. During glacial intervals, significant increases in intermediate-to-deep δ13C gradients (commonly reaching >1.2‰) are consistent with changes in deep water circulation and associated chemical stratification. Bathymetric δ13C gradients covary with benthic foraminiferal δ18O and covary inversely with Vostok CO2, in agreement with chemical stratification as a driver of atmospheric CO2 changes. Three deep circulation indices based on δ13C show a phasing similar to North Atlantic sea surface temperatures, consistent with a Northern Hemisphere control of NADW/SOW variations. However, lags in the precession band indicate that factors other than deep water circulation control ice volume variations at least in this band

Stability of North Atlantic water masses in face of pronounced climate variability during the Pleistocene

Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 19 (2004): PA2008, doi:10.1029/2003PA000921.Geochemical profiles from the North Atlantic Ocean suggest that the vertical δ13C structure of the water column at intermediate depths did not change significantly between glacial and interglacial time over much of the Pleistocene, despite large changes in ice volume and iceberg delivery from nearby landmasses. The most anomalous δ13C profiles are from the extreme interglaciations of the late Pleistocene. This compilation of data suggests that, unlike today (an extreme interglaciation), the two primary sources of northern deep water, Norwegian-Greenland Sea and Labrador Sea/subpolar North Atlantic, had different characteristic δ13C values over most of the Pleistocene. We speculate that the current open sea ice conditions in the Norwegian-Greenland Sea are a relatively rare occurrence and that the high-δ13C deep water that forms in this region today is geologically unusual. If northern source deep waters can have highly variable δ13C, then this likelihood must be considered when inferring past circulation changes from benthic δ13C records.National Science Foundation grants OCE-0118005 and OCE-0118001, which supported MER and DWO

The diagnostic value of ultrasonography-derived edema of the temporal artery wall in giant cell arteritis: a second meta-analysis

<p>Abstract</p> <p>Background</p> <p>Ultrasonography of temporal arteries is not commonly used in the approach of patients with suspected giant cell arteritis (GCA) in clinical practice. A meta-analysis of primary studies available through April 2004 concluded that ultrasonography could indeed be helpful in diagnosing GCA. We specifically re-examined the diagnostic value of the ultrasonography-derived halo sign, a dark hypoechoic circumferential thickening around the artery lumen, indicating vasculitic wall edema, in GCA.</p> <p>Methods</p> <p>Original, prospective studies in patients with suspected GCA that examined ultrasonography findings of temporal arteries using the ACR 1990 classification criteria for GCA as reference standard, published through 2009, were identified. Only eight studies involving 575 patients, 204 of whom received the final diagnosis of GCA, fulfilled technical quality criteria for ultrasound. Weighted sensitivity and specificity estimates of the halo sign were assessed, their possible heterogeneity was investigated and pooled diagnostic odds ratio was determined.</p> <p>Results</p> <p>Unilateral halo sign achieved an overall sensitivity of 68% (95% CI, 0.61-0.74) and specificity of 91% (95% CI, 0.88-0.94) for GCA. The values of inconsistency coefficient (I²) of both sensitivity and specificity of the halo sign, showed significant heterogeneity concerning the results between studies. Pooled diagnostic odds ratio, expressing how much greater the odds of having GCA are for patients with halo sign than for those without, was 34 (95% CI, 8.21-138.23). Diagnostic odds ratio was further increased to 65 (95% CI, 17.86-236.82) when bilateral halo signs were present (sensitivity/specificity of 43% and 100%, respectively). In both cases, it was found that DOR was constant across studies.</p> <p>Conclusion</p> <p>Temporal artery edema demonstrated as halo sign should be always looked for in ultrasonography when GCA is suspected. Providing that currently accepted technical quality criteria are fulfilled, halo sign's sensitivity and specificity are comparable to those of autoantibodies used as diagnostic tests in rheumatology. Validation of revised GCA classification criteria which will include the halo sign may be warranted.</p

Atlantic Deep-water Response to the Early Pliocene Shoaling of the Central American Seaway

The early Pliocene shoaling of the Central American Seaway (CAS), ~4.7–4.2 million years ago (mega annum-Ma), is thought to have strengthened Atlantic Meridional Overturning Circulation (AMOC). The associated increase in northward flux of heat and moisture may have significantly influenced the evolution of Pliocene climate. While some evidence for the predicted increase in North Atlantic Deep Water (NADW) formation exists in the Caribbean and Western Atlantic, similar evidence is missing in the wider Atlantic. Here, we present stable carbon (δ13C) and oxygen (δ18O) isotope records from the Southeast Atlantic-a key region for monitoring the southern extent of NADW. Using these data, together with other δ13C and δ18O records from the Atlantic, we assess the impact of the early Pliocene CAS shoaling phase on deep-water circulation. We find that NADW formation was vigorous prior to 4.7 Ma and showed limited subsequent change. Hence, the overall structure of the deep Atlantic was largely unaffected by the early Pliocene CAS shoaling, corroborating other evidence that indicates larger changes in NADW resulted from earlier and deeper shoaling phases. This finding implies that the early Pliocene shoaling of the CAS had no profound impact on the evolution of climate

Late Neogene history of deepwater ventilation in the Southern Ocean

We compiled carbon isotope records from the North Atlantic, South Atlantic, and Pacific oceans to estimate changes in vertical and interbasinal δ13C gradients for the last 9 Myr. Benthic δ13C values of deep water in the South Atlantic decreased in a series of steps at ∼7, 2.75, and 1.55 Ma away from the North Atlantic and toward the Pacific. The benthic δ13C of intermediate water in the South Atlantic evolved differently from deep waters, resulting in an increase in the intermediate-to-deep δ13C gradient (Δ13CID). The Δ13CID increased in steps at ∼2.75 and 1.55 Ma, marking the development and intensification of a chemical divide in the Atlantic Ocean between well-ventilated intermediate water and poorly ventilated deep water. We suggest these changes in interbasinal and vertical gradients resulted from decreasing Northern Component Water and reduced ventilation of Southern Component Water (SCW). The latter was caused by several interrelated processes in Antarctic sources areas, including increased sea ice cover, enhanced surface water stratification, decreased Ekman-induced upwelling, and reduced vertical mixing across the thermocline. Because Antarctic surface water processes and deepwater ventilation rates ultimately influence atmospheric CO2 concentration, these processes may have acted as a positive feedback that contributed to the major cooling steps in late Neogene climate

Late Neogene history of deepwater ventilation in the Southern Ocean

We compiled carbon isotope records from the North Atlantic, South Atlantic, and Pacific oceans to estimate changes in vertical and interbasinal δ13C gradients for the last 9 Myr. Benthic δ13C values of deep water in the South Atlantic decreased in a series of steps at ∼7, 2.75, and 1.55 Ma away from the North Atlantic and toward the Pacific. The benthic δ13C of intermediate water in the South Atlantic evolved differently from deep waters, resulting in an increase in the intermediate-to-deep δ13C gradient (Δ13CID). The Δ13CID increased in steps at ∼2.75 and 1.55 Ma, marking the development and intensification of a chemical divide in the Atlantic Ocean between well-ventilated intermediate water and poorly ventilated deep water. We suggest these changes in interbasinal and vertical gradients resulted from decreasing Northern Component Water and reduced ventilation of Southern Component Water (SCW). The latter was caused by several interrelated processes in Antarctic sources areas, including increased sea ice cover, enhanced surface water stratification, decreased Ekman-induced upwelling, and reduced vertical mixing across the thermocline. Because Antarctic surface water processes and deepwater ventilation rates ultimately influence atmospheric CO2 concentration, these processes may have acted as a positive feedback that contributed to the major cooling steps in late Neogene climate

Climate drying and associated forest decline in the lowlands of northern Guatemala during the late Holocene

Palynological studies document forest disappearance during the late Holocene in the tropical Maya lowlands of northern Guatemala. The question remains as to whether this vegetation change was driven exclusively by anthropogenic deforestation, as previously suggested, or whether it was partly attributable to climate changes. We report multiple palaeoclimate and palaeoenvironment proxies (pollen, geochemical, sedimentological) from sediment cores collected in Lake Petén Itzá, northern Guatemala. Our data indicate that the earliest phase of late Holocene tropical forest reduction in this area started at ~ 4500 cal yr BP, simultaneous with the onset of a circum-Caribbean drying trend that lasted for ~ 1500 yr. This forest decline preceded the appearance of anthropogenically associated Zea mays pollen. We conclude that vegetation changes in Petén during the period from ~ 4500 to ~ 3000 cal yr BP were largely a consequence of dry climate conditions. Furthermore, palaeoclimate data from low latitudes in North Africa point to teleconnective linkages of this drying trend on both sides of the Atlantic Ocean