Richard W. Murray

Contrasting sulfur geochemistry and Fe/Al and Mo/Al ratios across the last oxic-to-anoxic transition in the Cariaco Basin, Venezuela

An abrupt transition from oxic to anoxic-sulfidic (euxinic) marine bottom waters occurred in the Cariaco Basin in response to increasing productivity resulting from the late Pleistocene post-glacial rise in sea level and corresponding increase in surface- water nutrient availability. The microlaminated sediments of the euxinic interval, which span the last f14.5 ky, suggest a predominance of water-column (syngenetic) pyrite formation based on (1) high pyrite sulfur (Spy) concentrations in the surficial sediment layers, (2) values for degree-of-pyritization (DOP) that generally do not increase appreciably with increasing burial, (3) ratios of total iron (FeT) to Al that are elevated above the continental baseline recorded in the underlying oxic sediments, and (4) Spy isotope trends that largely mimic the d 34 SHSof the modern water column. Intermediate DOP values in the microlaminated deposits and FeT/Al ratios that are slightly above continental levels indicate an iron reservoir controlled by scavenging during syngenetic pyrite formation in combination with intermediate rates of Fe-bearing siliciclastic accumulation. As predicted from the relative rates of siliciclastic delivery, FeT/Al and DOP data lie between end-member values observed in the modern Black Sea. As viewed broadly, FeT/Al and DOP relationships in euxinic sediments reflect the balance between syngenetic Fe scavenging and temporal and spatial gradients in siliciclastic input. Pyrite concentrations are generally low in the underlying oxic marine deposits because of limitations in the supply of organic carbon (Corg). However, the upper 80 cm of the Fe-rich, Corg-poor, bioturbated sediment show evidence for a strong diffusional HSoverprint from the overlying, Fe-limited euxinic marine environment. This post-glacial transition manifests in pyrite overprints that are strongly 34 S-depleted relative to those in restricted, presently euxinic marine settings elsewhere in the world, such as the Black Sea, where the sedimentary sequence spanning the last glacial-interglacial transition begins with a shift from freshwater to Corg-poor oxic marine deposition and thus dominantly sulfate diffusion. Trends for Mo/Al ratios in the microlaminated sediments suggest that Mo is enriched by roughly two orders of magnitude above the continental levels recorded in the oxic deposits. Organic matter plays a role by enhancing HSproduction and/or by providing a substrate for Mo scavenging. Significant Mo enrichment via diffusion into the upper portion of the bioturbated zone was not observed despite HS � -rich pore waters as recorded in the heavy iron sulfide overprint. We have

SCAVENGED EXCESS ALUMINUM AND ITS RELATIONSHIP TO BULK TITANIUM IN BIOGENIC SEDIMENT FROM THE CENTRAL EQUATORIAL PACIFIC OCEAN

We present results from chemical analyses of Al and Ti in surface sediment sampled along two cross-Equator latitudinal transects at 135W and 140W in the central equatorial Pacific Ocean. Although traditionally both Al and Ti are considered to reside exclusively within terrigenous phases in marine sediment, these sediments present extremely high Al/Ti ratios that are several times that of average shale and other potential crustal sources. A sharp maximum in Al/Ti is observed slightly south of the Equator, where sedimentary bulk accumulation rate (BAR) is also highest (reflecting elevated productivity in the overlying water caused by surface water divergence). Bulk Al/Ti decreases sharply away (±2° latitude) to near crustal values at ∼4° north and south. The latitudinal profiles of Al/Ti are entirely unrelated to the concentration of the biogenic components as well as to the absolute accumulation of Al and Ti. These results indicate the presence of a significant scavenged component of Al sourced directly from seawater during particle settling. The data from the two transects analytically and oceanographically confirms our earlier work that was based on the single 135W transect. Calculations of Alexcess indicate that the highest Al/Ti ratios correspond to ∼50% of the total Al being unsupported by the small amount of terrigenous phases present. These results are consistent with previous and ongoing studies of biogenic sediment, suspended particulate matter, and sediment trap material. Quantitative use of Al as an index of terrigenous material may, therefore, lead to an overestimation, by a factor of two, of the true terrigenous load in marine sediment, sedimentary rock, and settling particles. Because bulk Al/Ti appears to respond to sedimentary BAR, which in biogenic regimes is linked to surface water productivity, downcore records of Al/Ti in biogenic sediment may track productivity changes through time. Such Al/Ti proxy records may be applicable in sediment of all ages, unlike radionuclide tracers which are limited by radioactive decay to use over the past hundreds of kyr.

Biogenic flux of Al to sediment in the central equatorial Pacific Ocean: Evidence for increased productivity during glacial periods

We examined the flux of Al to sediment accumulating beneath the zone of elevated productivity in the central equatorial Pacific Ocean, along a surface sediment transect at 135°W as well as downcore for a 650 kyr record at 1.3°N, 133.6°W. Across the surface transect, a pronounced, broadly equatorially symmetric increase in Al accumulation is observed, relative to Ti, with Al/Ti ratios reaching values 3–4 times that of potential detrital sources. The profile parallels biogenic accumulation and the modeled flux of particulate 234Th, suggesting rapid and preferential adsorptive removal of Al from seawater by settling biogenic particles. Normative calculations confirm that most Al is unsupported by the terrigenous fraction. The observed distributions are consistent with previous observations of the relative and absolute behavior of Al and Ti in seawater, and we can construct a reasonable mass balance between the amount of seawater-sourced Al retained in the sediment and the amount of seawater Al available in the overlying column. The close tie between Al/Ti and biogenic accumulation (as opposed to concentration) emphasizes that biogenic sedimentary Al/Ti responds to removal-transport phenomena and not bulk sediment composition. Thus, in these sediments dominated by the biogenic component, the bulk Al/Ti ratio reflects biogenic particle flux, and by extension, productivity of the overlying seawater. The downcore profile of Al/Ti at 1.3°N displays marked increases during glacial episodes, similar to that observed across the surface transect, from a background value near Al/Ti of average upper crust. The excursions in Al/Ti are stratigraphically coincident with maxima in both bulk and CaCO3 accumulation and the excess Al appears to not be preferentially affiliated with opaline or organic phases. Consistent with the similar behavioral removal of Al and 234Th, the latter of which responds to the total particle flux, the Al flux reflects carbonate accumulation only because carbonate comprises the dominant flux in these particular deposits. These results collectively indicate that (1) Al in biogenic sediment and settling biogenic particles is strongly affected by a component adsorbed from seawater. Therefore, the common tenet that Al is dominantly associated with terrestrial particulate matter, and the subsequent use of Al distributions to calculate the abundance and flux of terrestrial material in settling particles and sediment, needs to be reevaluated. (2) The Al/Ti ratio in biogenic sediment can be used to trace the productivity of the overlying water, providing a powerful new paleochemical tool to investigate oceanic response to climatic variation. (3) The close correlation between the Al/Ti productivity signal and carbonate maxima downcore at 1.3°N suggests that the sedimentary carbonate maxima in the central equatorial Pacific Ocean record increased productivity during glacial episodes.

A comparison between excess barium and barite as indicators of carbon export

[1] Since Dymond et al. [1992] proposed the paleoproductivity algorithm based on ‘‘Bio-Ba,’’ which relies on a strong correlation between Ba and organic carbon fluxes in sediment traps, this proxy has been applied in many paleoproductivity studies. Barite, the main carrier of particulate barium in the water column and the phase associated with carbon export, has also been suggested as a reliable paleoproductivity proxy in some locations. We demonstrate that Baexcess (total barium minus the fraction associated with terrigenous material) frequently overestimates Babarite (barium associated with the mineral barite), most likely due to the inclusion of barium from phases other than barite and terrigenous silicates (e.g., carbonate, organic matter, opal, Fe-Mn oxides, and hydroxides). A comparison between overlying oceanic carbon export and carbon export derived from Baexcess shows that the Dymond et al. [1992] algorithm frequently underestimates carbon export but is still a useful carbon export indicator if all caveats are considered before the algorithm is applied. Babarite accumulation rates from a wide range of core top sediments from different oceanic settings are highly correlated to surface ocean 14 C and Chlorophyll a measurements of primary production. This relationship varies by ocean basin, but with the application of the appropriate f ratio to 14 C and Chlorophyll a primary production estimates, the plot of Babarite accumulation and carbon export for the equatorial Pacific, Atlantic, and Southern Ocean converges to a global relationship that can be used to reconstruct paleo carbon export. INDEX TERMS: 3022 Marine Geology and Geophysics: Marine sediments—processes and transport; 4267 Oceanography: General: Paleoceanography; 4825 Oceanography: Biological and Chemical: Geochemistry; KEYWORDS: paleoproductivity, barite, export production, excess barium, marine sediments Citation: Eagle, M., A. Paytan, K. R. Arrigo, G. van Dijken, and R. W. Murray, A comparison between excess barium and barite as indicators of carbon export, Paleoceanography, 18(1), 1021, doi:10.1029/2002PA000793, 2003.

Barium in equatorial Pacific carbonate sediment: Terrigenous, oxide, and biogenic associations

We have analyzed carbonate sediment from Ocean Drilling Program Site 850 (equatorial Pacific Ocean) in order to assess the changing effects of sedimentation style on Ba accumulation through time. Formed along the East Pacific Rise ∼12 m.y. ago at 1°–2°S and now located at 1.3°N, Site 850 has experienced many changes through its northwesterly migration, including a crossing of the equator at ∼4 Ma. We divide sedimentation into three stages (phases I, II, and III; with phase III being youngest) according to geographic position, lithostratigraphy, and bulk accumulation rate. Of greatest importance are contrasts between phase II (7.5–4.0 m.y. ago) and phase III (4.0–0 m.y. ago). Phase II includes the previously described “biogenic bloom” as well as the depositional record of elevated productivity near the equator. In phase II the accumulation of Ba shows the strongest correlation with the accumulation of CaCO3 (r² = 0.69), opal (r² = 0.44), and Corg (r² = 0.41) compared with elsewhere through the sequence, although the correlation with terrigenous accumulation is also very strong (r² = 0.63). In phase III, which records deposition in the northern hemisphere and is thus closer to terrigenous input at the Intertropical Convergence Zone (ITCZ), there are no statistically significant correlations between the accumulations rates of Ba and CaCO3, Ba and opal, and Ba and Corg. Most significantly, through phase III the Ba accumulation rate is extremely strongly tied to terrigenous accumulation (r² = 0.89), which is the strongest correlation of any in our database, and to the accumulation of Feexcess (r² = 0.74), which we use to track the Fe-oxide component. Cross-equator surface sediment transects have previously indicated that beneath the ITCZ there is a pronounced local maximum in Ba accumulation, and the strong tie between Ba and terrigenous accumulation and associated Fe-oxides at Site 850 also corresponds with its tectonic migration toward the ITCZ. We conclude that the putative link between Ba accumulation and export production may be obscured by changes in particle composition even within a dominantly biogenic sedimentary regime, as well as by early diagenetic transfer between phases, and that the non-barite elemental Ba inventory may complicate the use of elemental Ba as a quantitative proxy for barite in the bulk sediment.

Export production and carbonate dissolution in the central equatorial Pacific Ocean over the past 1 Myr

In order to quantify changes in export production and carbonate dissolution over the past 1 Myr in the central equatorial Pacific Ocean we analyzed Ba, P, A1, Ti, and Ca in 1106 samples from five piston cores gathered from 5oS to 4oN at 140oW. We focused on Ba/Ti, A1/Ti, and P/Ti ratios as export proxies and employed areally integrated time slice as well as time series strategies. Carbonate maxima from 0-560 kyr are characterized by 15-30% greater export than carbonate minima. The increases in export fall on glacial /5180 transitions rather than glacial maxima. From 560-800 kyr, overlapping with the mid-Pleistocene transition, there is a very large increase in total export yet no glacial-interglacial variability. The highest latitudes (5oS and 4oN) record minimal absolute export change from glacials to interglacials and yet record the most extreme minima in percent CaCO3, indicating that carbonate records there are dominated by dissolution, whereas near the equator they are more influenced by changes in export.

Decoupling of carbonate preservation, carbonate concentration, and biogenic accumulation: A 400‐kyr record from the central equatorial Pacific Ocean

In order to investigate the paleoceanographic record of dissolution of calcium carbonate (CaCO3) in the central equatorial Pacific Ocean, we have studied the relationship between three indices of foraminiferal dissolution and the concentration and accumulation of CaCO3, opal, and Corg in Core WEC8803B-GC51 (1.3°N, 133.6°W; 4410 m). This core spans the past 413 kyr of deposition and moved in and out of the lysoclinal transition zone during glacial-interglacial cycles of CaCO3 production and dissolution. The record of dissolution intensity provided by foraminiferal fragmentation, the proportion of benthic foraminifera, and the foraminiferal dissolution index consistently indicates that the past corrosion of pelagic CaCO3 in the central equatorial Pacific does not vary with the observed sedimentary concentration of CaCO3. Although there is a weak low-frequency variation (∼100 kyr) in dissolution intensity, it is unrelated to sedimentary CaCO3 concentration. There are many shorter-lived episodes where high CaCO3 concentration is coincident with poor foraminiferal preservation, and where, conversely, low CaCO3 concentration is coincident with superb foraminiferal preservation. Spectral analyses indicate that dissolution maxima consistently lagged glacial maxima (manifest by the SPECMAP δ18O stack) in the 100-kyr orbital band. Additionally, there is no relationship between dissolution and the accumulation of biogenic opal or Corg or between dissolution and the burial ratio of Corg/CINorg (calculated from Corg and CaCO3). Because previous studies of this core strongly suggest that surface water productivity varied closely with CaCO3 accumulation, both the mechanistic decoupling of carbonate dissolution from CaCO3 concentration (and from biogenic accumulation) and the substantial phase shift between dissolution and global glacial periodicity effectively obscure any simple link between export production, CaCO3 concentration, and dissolution of sedimentary CaCO3.

Geochemical evidence for variations in delivery and deposition of sediment in Pleistocene light-dark color cycles under the Benguela Current Upwelling System

Abstract Distinctive light–dark color cycles in sediment beneath the Benguela Current Upwelling System indicate repetitive alternations in sediment delivery and deposition. Geochemical proxies for paleoproductivity and for depositional conditions were employed to investigate the paleoceanographic processes involved in creating these cycles in three mid-Pleistocene intervals from ODP Sites 1082 and 1084. Concentrations of total organic carbon (TOC) vary between 3.5 and 17.1%. Concentrations of CaCO3 vary inversely to TOC and Al, which suggests that both carbonate dissolution and terrigenous dilution contribute to the light–dark cycles. Opal concentrations are independent of both TOC and CaCO3, therefore eliminating diatom production and lateral transport of shelf material as causes of the light–dark cycles. δ13Corg and δ15Ntot values do not vary across light–dark sediment intervals, implying that the extent of relative nutrient utilization did not change. The stable δ15Ntot values represent a balanced change in nitrate supply and export production and therefore indicate that productivity was elevated during deposition of the TOC-rich layers. Parallel changes in concentrations of indicator trace elements and TOC imply that changes in organic matter delivery influenced geochemical processes on the seafloor by controlling consumption of pore water oxygen. Cu, Ni, and Zn are enriched in the darker sediment as a consequence of greater organic matter delivery. Redox-sensitive metals vary due to loss (Mn and Ba) or enrichment (Mo) under reducing conditions created by TOC oxidation. Organic matter delivery impacts subsequent geochemical changes such as carbonate dissolution, sulfate reduction and the concentration of metals. Thus, export production is considered ultimately responsible for the generation of the color cycles.

Elemental fractionation of Si, Al, Ti, Fe, Ca, Mn, P, and Ba in five marine sedimentary reference materials: results from sequential extractions

Abstract We report on the elemental results from sequential extractions of BCSS-1 (marine sediment), MESS-1 (estuarine sediment), MAG-1 (marine mud), SCo-1 (Cody shale), and NIST-1c (argillaceous limestone) to encourage future comparisons of sequential extraction results within the marine geochemical and paleoceanographic communities. We measured Si, Al, Ti, Fe, Mn, Ca, P, and Ba in sequential de-ionized water (loosely-bound), MgCl2 (exchangeable), acetic acid (carbonate), hydroxylammonium hydrochloride (oxide), H2O2 (organic), Na2CO3 (opal), and residual (lithogenic) leaches. The protocol and selected elements were tailored to be most relevant to paleoceanographic geochemical studies instead of to environmental studies. Our results show that the sequential extraction procedure faithfully yields elemental distributions that are consistent with individual SRM lithologies. Our results also show that the procedure is typically reproducible within approximately 15%. However, in almost all cases, the procedure suffers from a systematic under-recovery of material when compared with the certified, bulk chemical analysis, and the under-recovery appears to be related to the lithology of the SRM. Similar under-recovery appears to be typical of sequential extraction procedures as reported by other previous studies. While this is problematic in assessing closure, it does not diminish the potential of inter-lab comparisons and first-order accuracy comparisons. We found that the elemental totals for the sequential extractions of MAG-1 compared best with the certified, bulk totals, and we recommend using this SRM to facilitate future accuracy assessments and inter-lab comparisons.

Geochemical approaches to the quantification of dispersed volcanic ash in marine sediment

Volcanic ash has long been recognized in marine sediment, and given the prevalence of oceanic and continental arc volcanism around the globe in regard to widespread transport of ash, its presence is nearly ubiquitous. However, the presence/absence of very fine-grained ash material, and identification of its composition in particular, is challenging given its broad classification as an “aluminosilicate” component in sediment. Given this challenge, many studies of ash have focused on discrete layers (that is, layers of ash that are of millimeter-to-centimeter or greater thickness, and their respective glass shards) found in sequences at a variety of locations and timescales and how to link their presence with a number of Earth processes. The ash that has been mixed into the bulk sediment, known as dispersed ash, has been relatively unstudied, yet represents a large fraction of the total ash in a given sequence. The application of a combined geochemical and statistical technique has allowed identification of this dispersed ash as part of the original ash contribution to the sediment. In this paper, we summarize the development of these geochemical/statistical techniques and provide case studies from the quantification of dispersed ash in the Caribbean Sea, equatorial Pacific Ocean, and northwest Pacific Ocean. These geochemical studies (and their sedimentological precursors of smear slides) collectively demonstrate that local and regional arc-related ash can be an important component of sedimentary sequences throughout large regions of the ocean.