Article Google Scholar. Shelf Sci. Croudace, I. Archaeometry 30 , — Spain as inferred from the degrees of pyritization of iron and trace elements. Google Scholar. Howarth, R. Total Environ. CAS Google Scholar. Desprat, S. The shallow sill depth at Bosphorus caused the connection of the Black Sea with the world ocean system to be intermittently cut off especially during the glacial lowstands. In the last glacial epoch and deglaciation, the Black Sea was a fresh water lake, and a lacustrine clay unit Unit 3 was deposited Figure 2 [24, 66].
The last connection with the Mediterranean was established through the Bosphorus at 7, yr BP [70]. Following this connection, high organic productivity and restricted circulation conditions caused deposition of a sapropel unit Unit 2. With the invasion of the basin by the coccolithophore Emiliania Huxleyi at about yr BP [66] and the establishment of the present oceanographic conditions, a micro laminated coccolith mud Unit 1 started depositing in the basin.
The purpose of this paper is to overview these studies concerning the salient organic and inorganic geochemical characteristics of the late Quaternary Black Sea sediments, and together with some new data, relate these characteristics to the paleogeographic, oceanographic and biological evolution of the Black Sea. Om Unit 1 "Coccolith unit" "Recent" 0. Late Pleistocene-Holocene stratigraphy of the deep Black Sea basm 2.
Stratigraphy of the late Quaternary sediments The general stratigraphic and sedimentological features of the late Quaternary sediments in the Black Sea were studied extensively by the Soviet workers [1, 3, 79, 80] and by the RIV Atlantis II expedition [66, 67].
These studies clearly established the presence of three distinct stratigraphic units in the top 1 m in the abyssal plain Figure 2. These units have been deposited in the last 30, yr and named from top to bottom as Unit 1 coccolith , Unit 2 sapropel and Unit 3 lutite. They roughly correspond to the "Recent", "old Black Sea" and "Neoeuxinian" units of the Russian workers e.
The shelf areas are mostly composed of pelecypod, gastropod, foraminifer, coccolithophore and ostracod-bearing sand and mud [16].
These shelf sediments locally contain layers consisting of shell accumulations and black organic-rich lamina. Later, Jones [47] and Jones et al. Some workers used varve chronology to date the unit boundaries, assuming that one light and one dark laminae represent one year [26, 30, 41].
On the basis of varve chronology the ages obtained for the base of Unit 1 and Unit 2 in the deep basin range between and yr, respectively. Calvert et al. Therefore, the radiocarbon ages of and yr BP for the base of Unit 1 and Unit 2 are accepted in the present paper. Unit 1 is cm-thick coccolith marl, consisting of alternations of light- and dark- colored microlaminae. The light-colored laminae are composed mainly of calcareous coccolith remains.
The dark laminae consist of clays and organic matter. Unit 2 is cm thick sapropel, consisting mainly of gelatinous organic matter with some coccolith remains, clays, inorganically precipitated aragonite, iron monosulfides and pyrite. Unit 2 is occasionally interrupted by distal turbidite layers, which sometimes occur between Units 1 and 2 e. The sapropel unit was deposited during a period of high plankton productivity [18, 61] after the flooding of the lacustrine Black Sea basin by the Mediterranean waters via the Bosphorus strait at 7, yr BP [70].
It includes dark laminae that are formed by high concentrations of unstable iron mono-sulfides such as mackinawite and greigite [9]. Unit 3 was deposited under freshwater lacustrine conditions during ,, yr BP [29, 66] when the water level of the Black Sea was about m lower than the present sea level [70].
Geochemistry 3. J Organic carbon content and distribution The organic carbon C org content of the sapropel unit The surface sediments from all the physiographic regions of southern part of the Black Sea Basin contain an average of 2.
This distribution closely approximates that of the sedimentation rates given by Ross et al. The high C org values in Unit 2 are a result of the high organic productivity and organic mass accumulation rates in the Black Sea during the sapropel formation e. The relatively high C org content of Unit 1 is partly the result of high organic matter preservation under unoxic water-column conditions.
Data], l3C magnetic resonance spectrometry Ediger et aI. Data] and reflectance microscopy [33]. These various studies have often produced contradictory results concerning the origin of the organic matter in these sediments. The results of the organic geochemical studies for each stratigraphic unit are summarized below. A similar study by Pelet and Debyser [62], however, shows that the organic matter in this unit is of mixed origin, with the marine component being more dominant than the terrestrial one.
The primary CC 0. A common value CC This value is close to that used for shales 0. The SP B 0. While we have no way of really knowing if this ratio is representative of the lithogenic sites Fig.
Barium in this case contrasts with the offshore sites have substantial Ba enrichments over the detrital distribution of Mo, which is expected to covary with manga- signature as compared to those sites in the near shore region.
Despite the fact that the patterns of Ba and the central California margin Stns. CC have very little, Mn enrichment do not covary, both the Patton Escarpment and if any, biogenic Ba. We argue that these patterns are due to a San Clemente sites are locations having the highest sedimen- diagenetic overprint on the Ba signal where at the lower or- tary Mn and Ba concentrations. Thus, although Ba may be ganic carbon export sites, Ba is better preserved than at those influenced by metal oxide cycling at some of our locations, the sites having higher Ba export and lie within the oxygen mini- differences in the Ba and Mn distributions suggest that this mum zone.
If this interpretation were true then we would process is not the sole process governing Ba cycling in these expect that the sedimentary Ba:Al ratio should decrease with an sediments. More work would be required to determine how increasing organic carbon oxidation rate measured at our study much of the Ba signal might be associated with metal oxides at site, which it does Fig. This interpretation of Fig. Our solid reasonable proxy for organic carbon export in these environ- phase Ba data provide an opportunity to elucidate where Ba is ments.
Furthermore, if the primary factor reducing biogenic Ba well-preserved and where it is not. Again, the prediction of the preservation is suboxic diagenesis, then the organic carbon Table 5. Solid-phase metal:aluminum data for several sites within the California Borderlands region. Note the decrease in Ba:Al with depth at all sites below ;5—10 cm and the increase in U:Al with depth below ;5 cm.
Units are weight ratios. Note that concentration scales change from site to site. Also note that within the data, the pattern of decreasing Ba:Al with increasing U is consistent both in down-core profiles and among the various sites.
Sediment geochemistry of Ba oxidation rate and the bottom water oxygen concentration are both important parameters, thus we would not necessarily ex- pect a perfect correlation between the Ba:Al ratio and Cox. Another notable feature of sedimentary Ba in the southern California region is that each of the solid phase Ba:Al profiles exhibit a decrease with depth near the bottom 10 —15 cm Fig.
This down-core decrease in the Ba:Al ratio may be the result of a diagenetic remobilization of solid phase Ba. In the case of San Pedro basin, a low oxygen basin where a significant fraction of carbon diagenesis occurs via sulfate reduction, the Ba:Al ratio approaches that of the detrital signature e. However, as noted in this report, the burial efficiency is sensitive to both the detrital Ba:Al ratio and the mass accumulation rate—which are both difficult to constrain in this region.
The essence of this discussion is that Ba exhibits a lower preservation in the near-shore regions of the southern Califor- nia continental margin as compared to offshore regions, and it appears to be poorly preserved within the oxygen minimum zone of the central California margin. The horizontal gradient in the Ba:Al ratio is roughly coincident with the organic carbon oxidation rate— organic carbon export decreases offshore.
In addition the Ba:Al ratio decreases with increasing depth at each of the sites where data is available. Both patterns suggest a diagenetic remobilization of Ba in continental margin sedi- ments. We should also note that some of the solid phase results are not consistent with a steady-state interpretation of the porewater results suggesting that some of the porewater profiles are being influenced by porewater irrigation processes rather than a down-core authigenic precipitation process.
The fun- damental question is whether or not the conditions that lead to decreased Ba preservation in this region can be found elsewhere in the ocean, and if so is there a way to track or proxy the conditions where Ba may be a poor indicator of productivity?
Interpreting the Sedimentary Barium Record Assuming that there is a mechanism related to suboxic diagenesis that corrupts the Ba record, then it would be advan- tageous to identify unreliable records. In other words, we need a proxy for the high rates of organic carbon diagenesis or export that result in sulfate reduction.
Because U uptake by sediments is particularly sensitive to suboxic to anoxic diagen- esis Anderson, ; Klinkhammer and Palmer, ; and others , the authigenic accumulation of this element is our best candidate. Although the mechanisms that control the removal Fig. This mechanism could be a is 0.
Without the one high U flux value, r2 for the and Cochran, or via a process which depends on the relationship is 0. Note that for sites having high U concentrations there is a corresponding low Ba concen- genic uptake Church et al. While we recognize that tration, which is what we would have predicted based on our observa- these and other geochemical uncertainties could complicate the tions from the California Margin.
Note the glacial enrichments north of the polar front for all constituents except for Ba. The PaTh ratio is assumed to be a proxy for changes in particle flux. The basis for this assumption is that although both these isotopes are scavenged to sediments, Th is more particle reactive than Pa. This fact results in a fractionation between these isotopes that is a function of particle export. Anderson et al.
Ratioing sedimentary Ba to Th is based on the assumption that the ratio of Th accumulation to its production can be used to evaluate the degree of sediment focusing or winnowing at a given location.
Thus Th normalization provides a tool for correcting the Ba or any constituent for sediment transport processes e. Data are from Kumar Consult Kumar et al. Taken as a whole, however, the Cali- total Ba preservation will be compromised. We would expect high sedimentary higher ratios off-shore, and there is some preliminary evidence U concentrations where surface Ba concentrations are low.
The that the accumulation of authigenic U may serve as a proxy for two sites having the highest biogenic Ba contents have the the corruption of the Ba record. It is necessary, therefore, to lowest sedimentary U contents. However, these two sites also examine the Ba-U relationship in other margin environments to exhibit down-core increases in the U:Al ratio with correspond- test its utility as a diagentic proxy.
This data is available from ing decreases in the Ba:Al ratio. Another way of looking at the Arabian Sea where our expectation of high U at locations U-Ba relationships would be to compare the benthic U flux having low Ba concentrations is met Fig. Note that we with the sedimentary Ba:Al ratio. We would expect higher have included the California margin data in this figure. We fluxes of U into the sediments higher authigenic U accumula- have excluded the high Ba data to show the expanded region tion rates to correspond to lower Ba:Al ratios, with U fluxes around the approximate crustal values.
Some of the offset in the either near zero or out of the sediments for sites having higher two data sets Calif. This prediction is observed along the California because of there being different crustal U and Ba abundances margin Fig. Sediments in the Arabian Sea reach much Tanner Basin is undoubtedly enigmatic in our analyses— lower sedimentary Ba and higher U concentrations, thereby having both high Ba:Al ratios as well has high U:Al ratios.
The extending the data set to a greater dynamic range of Ba and U. We would like to thank M. Colbert, G. Elrod, T. Kilgore, E. Nowicki, and S. Point Sur and R. We are grateful to Jack Dymond for identifying corrupted Ba records; however, the extent of that letting us use his unpublished Arabian Sea data. Comments by Jack corruption per unit of U accumulation remains unknown. Dymond, Marta Torres, and three anonymous reviewers helped im- prove this manuscript.
Acta tion record in those sediments will likely suffer from poor 46, — Data for the Southern Ocean Th and Pa from the open ocean. Earth Planet. Here, north of the Anderson R. Boundary effects and large-scale chemical fractionation. However, the Ba record does not exhibit a pattern consistent Roy.
London A, The record of authigenic U, clearly Deep Sea Res. Alterna- Barnes C. Acta 57, — Barnett P. Thus, south of the for taking virtually undisturved samples from shelf, bathyal, and Polar front we would interpret the high Ba, low U record as abyssal sediments.
Acta 7, — Deep-Sea From this example it is clear that the conditions leading to a Res. Berelson W. The discussions are built from the latest work in biogeochemistry and microbial ecology. And in the tradition of Robert Berner's classic treatment of early diagenesis, Burdige handles reaction pathways and transport processes rigorously and quantitatively. This nicely written, well-illustrated survey suits both the reference shelf and the classroom.
Lyons, University of California, Riverside "This is undoubtedly a major contribution to the field. Topics Atmospheric Science and Climatology Geosciences. Burdige Burdige, D. Burdige, David J.. Burdige D. Copy to clipboard. Log in Register.
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