ALBERT

Description: Tropical South America is one of the three main centres of the global, zonal overturning circulation of the equatorial atmosphere (generally termed the 'Walker' circulation1). Although this area plays a key role in global climate cycles, little is known about South American climate history. Here we describe sediment cores and down-hole logging results of deep drilling in the Salar de Uyuni, on the Bolivian Altiplano, located in the tropical Andes. We demonstrate that during the past 50,000 years the Altiplano underwent important changes in effective moisture at both orbital (20,000-year) and millennial timescales. Long-duration wet periods, such as the Last Glacial Maximum—marked in the drill core by continuous deposition of lacustrine sediments—appear to have occurred in phase with summer insolation maxima produced by the Earth's precessional cycle. Short-duration, millennial events correlate well with North Atlantic cold events, including Heinrich events 1 and 2, as well as the Younger Dryas episode. At both millennial and orbital timescales, cold sea surface temperatures in the high-latitude North Atlantic were coeval with wet conditions in tropical South America, suggesting a common forcing.

Description: Long sediment cores recovered from the deep portions of Lake Titicaca are used to reconstruct the precipitation history of tropical South America for the past 25,000 years. Lake Titicaca was a deep, fresh, and continuously overflowing lake during the last glacial stage, from before 25,000 to 15,000 calibrated years before the present (cal yr B.P.), signifying that during the last glacial maximum (LGM), the Altiplano of Bolivia and Peru and much of the Amazon basin were wetter than today. The LGM in this part of the Andes is dated at 21,000 cal yr B.P., approximately coincident with the global LGM. Maximum aridity and lowest lake level occurred in the early and middle Holocene (8000 to 5500 cal yr B.P.) during a time of low summer insolation. Today, rising levels of Lake Titicaca and wet conditions in Amazonia are correlated with anomalously cold sea-surface temperatures in the northern equatorial Atlantic. Likewise, during the deglacial and Holocene periods, there were several millennial-scale wet phases on the Altiplano and in Amazonia that coincided with anomalously cold periods in the equatorial and high-latitude North Atlantic, such as the Younger Dryas.

Description: Laboratory determinations of the distribution coefficient of strontium in calcite (k Sr ) have confirmed the earlier results of Katz et al. (1972). In addition, we observed that at higher concentrations of strontium, values of k Sr were somewhat lowered. Chemical analyses of several deep-sea carbonate sediment sections and their associated porewaters (DSDP Sites 288, 289, 315, 317, and 357) as well as previously published data of other workers (DSDP Sites 116 and 305) have been used to demonstrate that these k Sr values are appropriate for use in diagenetic studies. We have successfully modeled the distribution of strontium in the pore waters and sediments. As a result, we conclude that recrystallization of these carbonates (and many other limestones as well) is essentially isochemical, the cement being derived from within the formation. Advective fluid flow through the rock is unnecessary to explain its lithification. Recrystallization of the bulk of the calcite in deep-sea sections is largely complete within the upper few hundred meters, but cementation is generally a later process. These results have important implications for the study of oxygen isotopic compositions of foraminifers and coccoliths in many sediments older than about 20 m.y. B.P. From correlations between changes in Sr/Ca and Mg/Ca ratios of recrystallized carbonates of DSDP Site 305 a distribution coefficient of magnesium in calcite sediments of 8.1 x 10- 4 ( nearly equal 5 degrees C) has been estimated. This value is many times smaller than previously published values.

Description: H2O2 was measured in the upper water column (0–200 m) along a west-east transect through the Equatorial Atlantic as part of the German SOLAS (Surface Ocean Lower Atmosphere) cruise Meteor 55 (M55). Vertical profiles of H2O2 showed characteristic exponential decay consistent with light profiles and rainwater inputs. Integrated (0–100 m) water column H2O2 inventories ranged from 1.1–8.9 mmol m−2 with the highest values in the Amazon Plume. H2O2 inventories were also higher at the Equatorial Upwelling and after heavy rain showers in the region of the Inter Tropical Convergence Zone (ITCZ). Analysis of rain water samples collected during the cruise gave a volume weighted mean of 10.8 μmol L−1 (range 1.5–22.3 μmol L−1). This work highlights the importance of rainwater as a major source for H2O2 in the surface waters under the ITCZ.

Description: Atmospheric iron and underway sea-surface dissolved (〈0.2 μm) iron (DFe) concentrations were investigated along a north-south transect in the eastern Atlantic Ocean (27°N/16°W-19°S/5°E). Fe concentrations in aerosols and dry deposition fluxes of soluble Fe were at least two orders of magnitude higher in the Saharan dust plume than at the equator or at the extreme south of the transect. A weaker source of atmospheric Fe was also observed in the South Atlantic, possibly originating in southern Africa via the north-easterly outflow of the Angolan plume. Estimations of total atmospheric deposition fluxes (dry plus wet) of soluble Fe suggested that wet deposition dominated in the intertropical convergence zone, due to the very high amount of precipitation and to the fact that a substantial part of Fe was delivered in dissolved form. On the other hand, dry deposition dominated in the other regions of the transect (73-97), where rainfall rates were much lower. Underway sea-surface DFe concentrations ranged 0.02-1.1 nM. Such low values (0.02 nM) are reported for the first time in the Atlantic Ocean and may be (co)-limiting for primary production. A significant correlation (Spearman's rho = 0.862, p〈0.01) was observed between mean DFe concentrations and total atmospheric deposition fluxes, confirming the importance of atmospheric deposition on the iron cycle in the Atlantic. Residence time of DFe in the surface waters relative to atmospheric inputs were estimated in the northern part of our study area (17 ± 8 to 28 ± 16 d). These values confirmed the rapid removal of Fe from the surface waters, possibly by colloidal aggregation. © 2003 Elsevier Ltd. All rights reserved.

Description: Antarctic sponges are commonly fouled by diatoms, sometimes so heavily as to occlude pores employed in filter feeding and respiration. This fouling becomes heavier during the annual summer microalgal bloom. Polar and non‐polar extracts of eight species of marine sponges from McMurdo Sound, Antarctica were assayed for cytotoxicity against sympatric fouling diatoms. To identify compounds potentially released by sponges as defenses against diatom biofouling, only fractions of crude extracts that were soluble in seawater or 2% methanol in seawater were assayed. Significant bioactivity was present in seven of the eight species. Both Mycale acerata and Homaxinella balfourensis displayed moderate levels of defense against diatoms even though they are not or are only weakly chemically defended against bacteria and predators. Calyx acuarius extracts, which do have antipredator and antibacterial effects, had no effect on diatoms except at levels many fold higher than present in the intact animal. These results strongly suggest some level of specificity for chemical defenses against diatom fouling in antarctic sponges.

Description: Measurements of dissolved (DFe) and total iron (TFe) in the upper water column are presented from the German SOLAS (Surface Ocean ‐ Lower Atmosphere Study) cruise (M55), along a west to east transect at 10°N, in the equatorial Atlantic in October/November 2002. Aerosol samples were collected simultaneously during this time and are used to estimate an iron flux to the surface waters. Resulting flux estimates combined with iron inventories in the near surface waters reveal extremely short fractional mean residence times (6–62 days) for total (dissolved and particulate) iron in waters directly under the path of Saharan dust plumes. These results suggest that individual dust storms can supply a significant amount of the present iron upper water column inventory which is subsequent rapidly removed by aggregation and sinking.