Diverse studies predict global expansion of Oxygen Minimum Zones (OMZs) as a consequence of anthropogenic global warming. While the observed dissolved oxygen concentrations in many coastal regions are slowly decreasing, sediment core paleorecords often show contradictory trends. This is the case for numerous high-resolution reconstructions of oxygenation in the Eastern Tropical North Pacific (ETNP). While major shifts in redox conditions of the ETNP are dominated by glacial-interglacial cycling, important fluctuations also occur in response to minor climatic and oceanographic perturbations. It is important to understand these scenarios of past redox variation, as they are the closest analog for near future climate and oceanographic change. We present recently collected sediment core proxy records from the Gulf of California in which we reproduce the variability of productivity and oxygenation of the ETNP OMZ during the past millennium. We emphasize paleoproductivity (C org , Ni, Ba excess ) and paleoredox indicators (Mo, Cd, V, U auth ) in sediment cores collected in Alfonso and La Paz basins and compare these OMZ records with other archives of the Eastern Pacific. Our findings indicate that the OMZ expanded in response to increased upwelling and productivity during cold intervals of the early 1400s, early 1500s, late 1600s, and early 1800s AD (evidenced by higher Ni, V, Cd, Mo, and U auth ). The most hypoxic times corresponded to the beginning of the Little Ice Age (expressed in elevated Mo). Significant OMZ contractions occurred around late 1300s, early 1700s, and late 1900s AD after reoxygenation events that were instigated by low productivity (lower Ni, V, Cd, Mo, and U auth ). The mechanisms that control decadal-to-centennial oxygen variability in the ETNP remain unidentified but are likely influenced by solar forcing not only driving migrations of the Intertropical Convergence Zone (ITCZ) but more importantly changes in the intensity of the Pacific Walker Circulation (PWC). During the Little Ice Age solar irradiance was at its lowest for the past millennium, which strengthened the PWC. This pattern contributed to more frequent La Niña-like conditions, which enhanced upwelling of nutrient-rich waters in the west coast of North America, driving productivity and reducing bottom oxygen levels, as seen in our ETNP records.
To asses the geomobility of cadmium, copper, iron, manganese, nickel, lead, and zinc in marine sediments near the Santa Rosalía copper smelter, which is located on the eastern coast of the Baja California Peninsula, sequential leaching was applied to sediment samples containing different levels of Cu: (1) uncontaminated or slightly contaminated (<55 mg kg⁻¹ Cu); (2) moderately contaminated (55-500 mg kg⁻¹ Cu); and (3) heavily contaminated (>500 mg kg⁻¹ Cu). Concentrations of Cd, Cu, Fe, Mn, Ni, Pb, and Zn in four fractions of the leachate (mobile fraction F1, relatively mobile fraction F2, associated with organic matter/sulphides fraction F3, and residual fraction F4) were measured by atomic absorption spectrophotometry (AAS). The sediments with Cu concentration <500 mg kg⁻¹ displayed prevalence of mobile acid-leachable fraction F1 and reducible fraction F2 for Cd, Cu, Mn, and Pb, whereas the relative contribution of fraction F3 was relatively low for all of the examined metals. Residual fraction F4 was highest (>65%) for Fe and Ni because both metals are associated with the crystalline matrix of natural sediments. The sediments heavily contaminated with Cu (>500 mg kg⁻¹) had dramatically increased percentages of Cu, Mn, Pb, and Zn, ranging on average from 63 to 81%, in the residual fraction. In the case of Cu, for example, the relative abundances of this element in the different fractions of such sediments followed this sequence: residual fraction F4 (76 ± 5%) >absorbed form and carbonates fraction F1 (15 ± 5%) >Fe and Mn oxyhydroxides fraction F2 (5 ± 2%) >fraction associated with organic matter and sulphides F3 (4.5 ± 3.9%). Copper, Pb, and Zn contents in each geochemical fraction of all samples were compared with sediment-quality guideline values ("effects range low" [ERL] and "effects range medium" [ERM]) to assess their possible negative effects on biota. Copper contents in mobile fractions F1 and F2, which were moderately contaminated with Cu, were higher than ERL but lower than ERM guideline values. For heavily contaminated sediments, Zn contents of mobile fractions F1 and F2 were higher than ERL but lower than ERM guideline values. The Cu content of fraction F1 was higher than ERM guideline values, whereas for fractions F2 and F3 copper content was higher than ERL guidelines but still lower than ERM guideline values.
The Chicxulub impact caused a crash in export productivity in much of the world's oceans which contributed to the extinction of 75% of marine species. In the immediate aftermath of the extinction, local export productivity was highly variable, with some sites, including the Chicxulub crater, recording elevated export production. The long-term transition back to more stable export productivity regimes has been poorly documented. Here, we present elemental abundances, foraminifer and calcareous nannoplankton assemblage counts, total organic carbon, and stable carbon isotopes from the Chicxulub crater to reconstruct long-term changes of productivity over the first 3 Myr of the Paleocene. We show that export production was elevated for the first 320 kyr of the Paleocene and then declined over the next ~900 kyr, remaining low thereafter. This interval is associated with fluctuations in water column stratification and terrigenous flux, but these variables are uncorrelated to export productivity. Instead, we suggest that the turnover in the phytoplankton community from a post-extinction assemblage dominated by picoplankton (which promoted nutrient recycling in the euphotic zone) to a more normal Paleocene pelagic community dominated by calcareous nannoplankton (which more efficiently removed nutrients from surface waters and led to oligotrophy) is responsible for the decline in export production in the southern Gulf of Mexico.
The geochemical mobility of arsenic and some metals in the fine fraction of 20 alluvial sediment samples collected along the thalweg of the San Antonio arroyo was assessed in the abandoned gold mining district of the semiarid southeastern Baja California peninsula. Acid-digested element concentrations were determined by treating the fine fraction of the sediments with concentrated HNO 3 via microwave heating. A separate leaching of the diluted acidsoluble As and some metals from the samples was carried out using 1 M HCl. The most reactive As phase was extracted from the subsamples using an ascorbic solution of Na citrate buffered at pH 8. The high abundance of sulphosalts (tennantite Cu 12 As 4 S 13 and proustite Ag 3 AsS 3 ) in the sediments from the head of the arroyo changes downstream to the predominance of arsenic trioxide (arsenolite As 2 O 3 ) and arsenate (scorodite FeAsO 4 Á2H 2 O). Acid-digested As concentrations were always found to be high (230-270 mg kg -1 ) from the San Antonio village until the end of the arroyo. The concentration of reactive As and its relative contribution increased from low values of 2.3 mg kg -1 and 1 %, respectively, at the upstream of the arroyo, to 191-220 mg kg -1 and 75-89 % in the middle and lower parts, and is probably a result of As release from the mineral sulfide fraction of the tailings. Metallic contaminants exhibited different behavior, with a peak of acid-digested and diluted acid-leached concentrations observed in the central part of the arroyo and an almost permanent geochemical mobility.
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