Hydrocarbons in age-dated sediment cores from two basins in the Southern California Bight

The distributions of hydrocarbons in sediment cores dated by ²¹⁰Pb (1845–1977) from San Pedro and San Nicolas Basins in the Southern California Bight have been determined by gas Chromatographic and combined gas chromatographic-mass spectrometric analysis. The chromatograms of the hydrocarbons contain peaks of resolved alkanes and cycloalkanes, as well as an unresolved complex mixture which decrease in content with increasing depth in both of the cores. The concentrations of o,p' andp,p'-DDE are highest in the top 50 mm of the San Pedro core section. The 45–50 mm segment of this core was deposited in the 1945–1950 period when DDT (the precursor of DDE compounds) came into common use. The relatively high content of hydrocarbons and DDE in this core is attributable to the proximity of the site to the San Pedro Harbor which receives petroleum residues from shipping, sewage outfalls and industrial effluents. The presence of only traces of DDE throughout the San Nicolas core, and the low hydrocarbon content are explainable by the greater distance of this basin from anthropogenic inputs and probably a greater rate of degradation of deposited organic matter during bioturbation. δ¹³C, δ¹⁵N and electron-spin resonance analyses of kerogens and humic substances in these cores, indicate that most of this organic matter in these sediments is of a marine origin.     

On the validity and use of planktonic foraminifera in the interpretation of past climatic changes from a study of deep-sea cores

A recent test has shown that planktonic foraminifera, living for at least part of their life in the surface waters of the oceans, are limited to certain temperature zones. Since their evolution has been relatively slow, their calcareous remains, which accumulate on the deep-sea floor, can be analysed in samples taken from cores and temperature oscillations can be traced into the past by the percentage of warm, temperate and cold forms. It has been demonstrated that a count of a thousand specimens in each sample is all that is necessary to obtain these results.     

Holocene climate variability in the Southern Ocean recorded in a deep-sea sediment core off South Australia

To understand Holocene climatic development and to determine drivers of climatic changes and climate variability, high-resolution marine proxy records are required from key oceanic locations. However, information on the Holocene climate development from the Southern Hemisphere is still rare and mainly based on terrestrial archives. Here, we present data with a high-resolution of circa 35 years from sediment cores taken east of the Great Australian Bight, where it is possible to determine Southern Ocean Holocene climate and the longer-term trends of the El Niño–Southern Oscillation (ENSO) conditions. For this purpose, we used the oxygen-isotope records of two planktonic foraminifer species Globigerinoides ruber and Globigerina bulloides which inhabit different water masses as well as faunal counts of planktonic foraminifers. After the ocean frontal systems off southern Australia were pushed northward by orbitally-forced insolation changes during the early Holocene, the data indicate increasing ENSO variability during the mid to late Holocene when the fronts shifted polewards again. A strong circa 1550 year cycle is found in the Globigerina bulloides record which reflects the wider Southern Ocean signal with prominent cold phases centred at circa 9.2, 7.3, 5.8, 4.3, 2.7, 1.4 ka BP and, possibly the Little Ice Age, which have global counterparts.     

Response of deep-sea ostracod assemblages to Late Cretaceous palaeoceanographical changes: ODP Site 689 in the Southern Ocean

Twenty-six deep-sea ostracod species are described from the late Campanian to late Maastrichtian of ODP Site 689 in the Southern Ocean. Three are described as new:Cytherelloidea megaspirocostata,Agulhasina sudoceanicaandPennyella foveolata. Correspondence analysis identified three faunal sample groups. The first is mainly characterised byA. sudoceanica,Kirthesp. andCytherellasp. This is replaced, first temporarily (betweenc. 71 and 69 Ma) by a group mainly characterised byArgilloeciaspp. andC. megaspirocostata, possibly a consequence of a short-lived global palaeoceanographic event marked by increased ventilation and cooling of intermediate waters at site 689. It then reappears before finally being replaced by a third group largely characterised byEucytherecf.circumcostata,P. foveolataandDutoitella mimicaDingle. This final change is coeval with a marked increase in the faunal density of ostracods, probably owing to an increase in food supply. However, the oxygen isotopic records of benthonic foraminifera suggests that the replacement of the first ostracod group by the third is a result of the long term Maastrichtian cooling of intermediate waters at high latitudes. This replacement is also coeval with a gradual change in benthonic foraminiferal assemblages at site 689. It is suggested, therefore, thatPennyellaandDutoitellahad a greater potential of adapting to successively colder palaeoceanographical conditions thanAgulhasina. The former genera have a cosmopolitan deep-sea distribution today.     

Mercury in the Southern Ocean

We present here the first mercury speciation study in the water column of the Southern Ocean, using a high-resolution south-to-north section (27 stations from 65.50°S to 44.00°S) with up to 15 depths (0-4440 m) between Antarctica and Tasmania (Australia) along the 140°E meridian. In addition, in order to explore the role of sea ice in Hg cycling, a study of mercury speciation in the “snow-sea ice-seawater” continuum was conducted at a coastal site, near the Australian Casey station (66.40°S; 101.14°E). In the open ocean waters, total Hg (Hg_T) concentrations varied from 0.63 to 2.76 pmol L⁻¹ with “transient-type” vertical profiles and a latitudinal distribution suggesting an atmospheric mercury source south of the Southern Polar Front (SPF) and a surface removal north of the Subantartic Front (SAF). Slightly higher mean Hg_T concentrations (1.35 ± 0.39 pmol L⁻¹) were measured in Antarctic Bottom Water (AABW) compared to Antarctic Intermediate water (AAIW) (1.15 ± 0.22 pmol L⁻¹). Labile Hg (Hg_R) concentrations varied from 0.01 to 2.28 pmol L⁻¹, with a distribution showing that the Hg_T enrichment south of the SPF consisted mainly of Hg_R (67 ± 23%), whereas, in contrast, the percentage was half that in surface waters north of PFZ (33 ± 23%). Methylated mercury species (MeHg_T) concentrations ranged from 0.02 to 0.86 pmol L⁻¹. All vertical MeHg_T profiles exhibited roughly the same pattern, with low concentrations observed in the surface layer and increasing concentrations with depth up to an intermediate depth maximum. As for Hg_T, low mean MeHg_T concentrations were associated with AAIW, and higher ones with AABW. The maximum of MeHg_T concentration at each station was systematically observed within the oxygen minimum zone, with a statistically significant MeHg_Tvs Apparent Oxygen Utilization (AOU) relationship (p < 0.001). The proportion of Hg_T as methylated species was lower than 5% in the surface waters, around 50% in deep waters below 1000 m, reaching a maximum of 78% south of the SPF. At Casey coastal station Hg_T and Hg_R concentrations found in the “snow-sea ice-seawater” continuum were one order of magnitude higher than those measured in open ocean waters. The distribution of Hg_T there suggests an atmospheric Hg deposition with snow and a fractionation process during sea ice formation, which excludes Hg from the ice with a parallel Hg enrichment of brine, probably concurring with the Hg enrichment of AABW observed in the open ocean waters. Contrastingly, MeHg_T concentrations in the sea ice environment were in the same range as in the open ocean waters, remaining below 0.45 pmol L⁻¹. The MeHg_T vertical profile through the continuum suggests different sources, including atmosphere, seawater and methylation in basal ice. Whereas Hg_T concentrations in the water samples collected between the Antarctic continent and Tasmania are comparable to recent measurements made in the other parts of the World Ocean (e.g., Soerensen et al., 2010), the Hg species distribution suggests distinct features in the Southern Ocean Hg cycle: (i) a net atmospheric Hg deposition on surface water near the ice edge, (ii) the Hg enrichment in brine during sea ice formation, and (iii) a net methylation of Hg south of the SPF.     

太平洋富稀土深海沉积物中稀土元素赋存载体研究

太平洋深海沉积物中富含稀土(含Y,简称REY),尤其是(含)沸石粘土,其主要由粘土组分、沸石、鱼牙骨、微结核及生物残渣等组成,目前对于该类稀土矿床中REY的赋存载体存在争议。本文在中、西太平洋选取两个富REY的沸石粘土样品利用地球化学和矿物学对稀土赋存状态进行定量研究。矿物微区成分表明,鱼牙骨主要成分为磷灰石,含有最高的REY含量,为2497×10~(- 6)~18623×10~(- 6),微结核和沸石颗粒含有较低的REY含量,分别为246×10~(- 6)~333×10~(- 6)和29.36×10~(- 6)~256×10~(- 6)。通过三种矿物相在沉积物63~250μm粒级组分中各自所占质量比例,计算得出磷灰石对REY的贡献率可达90%以上,说明此粒级中磷灰石为主要REY赋存载体,然而相对全岩总的REY含量,该粒级中磷灰石的贡献仍较小。通过对全岩和粉砂级组分(小于63μm)主微量地球化学分析和XRD矿物相分析表明,2个样品中REY主要存在于粉砂级组分中,其中的磷灰石对全岩REY的贡献最高均可达70%左右。另外通过对粘土组分(2μm)研究发现2个样品粘土组分的REY含量相似,分别为530×10~(- 6)和631×10~(- 6),粘土组分对全岩沉积物的REY贡献意义不大,对整个沉积物REY而言其贡献率仅为2%~5%左右。因此认为磷灰石为整个沸石粘土中REY主要赋存载体。     

中太平洋PC5222孔岩芯地球化学特征及物源探讨

通过对中太平洋PC5222孔45个岩芯样品进行常微量元素的测试分析与研究,发现元素含量在垂向上有4次比较明显的变化。早期沉积物中,随着Al₂O₃、Fe₂O₃、K₂ O、MnO、TiO₂、CaO、CaCO₃含量的增加,SiO₂含量呈递减趋势;晚期变化不明显。元素的相关性分析和R型因子分析表明,Al₂ O₃、Fe₂ O₃、K₂ O、MnO、TiO₂之间存在明显的正相关关系,而与SiO₂为负相关。前者主要受陆源碎屑沉积的影响,后者与生物作用及火山活动有关。进一步探讨了晚渐新世沉积环境的演变规律。     

Sedimentological and 14C dating studies of past tsunami events in Southern Sri Lanka

After the 2004 Sumatra?CAndaman tsunamigenic earthquake, waters from the ocean moved upstream along rivers, bays, harbors, and lagoons and inundated many coastal and inland locations in the southern, eastern, and northern parts of Sri Lanka. The tsunami waters were observed to move upwards inland and then recede downwards to the ocean after varying inundation periods in different coastal areas. Subsequent massive tsunami waves came with the wave height varying from 3 to 8?m inland with speed of about 30?C40?kmph. The oceanic waves carrying heterogeneous sediments with water deposited them in coastal as well as inland locations about 1?km from the present coastline. Given the chaotic nature of tsunami oceanic waves, pre-tsunami deposits, such as beach sands, debris from coral reefs and buildings, parts of vehicles and ships, and tree trunks are found incorporated in authentic tsunami sediments. Thus, the texture, structure, and composition of sediments deposited by tsunami waters differed from one location to another. Therefore, in identifying paleo-tsunami sediments, care was taken to compare them with diagnostic unmixed uncontaminated recent tsunami sediments having characteristic textures and marine microfossil assemblages, such as foraminifera, radiolarians, and diatoms where preserved in coastal depressions. The radiocarbon ages of the carbonate and the organic fractions of these sediments are stratigraphically inconsistent, indicating mixing of sediments by the tsunami waves. The concentrations of organic carbon and nitrogen and their isotopic signatures confirm marine origin of these sediments.     

Pelagic records from the Equatorial Ninetyeast Ridge and significant environmental events during the past 3.5 Ma

This paper presents pelagic records of planktic foraminifera, as well as data of stable isotope stratigraphy and carbonate stratigraphy since 3.5 Ma B.P. from site ODP758 in the Ninetyeast Ridge of the Indian Ocean. Based on these data, manifestations and related mechanisms of major tectonic and environmental events such as the rapid uplift of the Himalaya Mountains, “middle Pleistocene climatic transition” and “mid-Brunhes dissolution event” in the region are discussed. According to the analysis and comparison of various indices and changes in terms of foraminifera assemblage, paleotemperature, paleosalinity and themocline from site ODP758, the authors deduce that the paleoclimatic changes might correlate with the mid-Pleistocene transition at 1.4–1.7 Ma B.P. The changes of CaCO₃, mass accumulation rates (MAR) of CaCO₃ and non- CaCO₃ MAR indicate that the loaded terrigenous sediments increased at 1.7 Ma, which is in agreement with the uplift history of the Qinghai-Tibet plateau as shown by the available data. The last two changes coincide with the uplift of the Qinghai-Tibet plateau, hence they are called “Qinghai-Tibet movement” (1.7 Ma), and the “Kunlun-Yellow River movement” (1.2–0.6 Ma). The changes of the CaCO₃ content, coarse fraction (> 150 μm) content and planktonic foraminifera biostratigraphy show that strong dissolution of abyssal CaCO₃ occurred in the study region during 0.5–0.4 Ma. The event was consistent with the “mid-Brunhes dissolution event” in the sedimentary records of the Atlantic Ocean, Pacific Ocean, Indian Ocean and Nansha sea area of the South China Sea. __________ Translated from Geological Bulletin of China, 2007, 26(12): 1627–1632 [译自: 地质通报]     

过去500年华南地区冷暖变化记录及其对冬季温度变化的定量指示意义

整编了历史文献中记载的1400年以来华南地区霜冻灾害及冰冻灾害南界、降雪南界及日数和初／终霜冻灾害日期记录; 并结合1951年以来器测气温序列, 分析了各类记录与华南地区11~2月平均气温的相关性, 揭示其对温度变化的定量指示意义。结果表明: 华南地区的霜冻灾害南界、冰冻灾害南界、降雪南界和降雪日数可较好地指示该区域的温度变化; 且24°N以南地区(包括广东、广西、台湾中南部和海南)的最早与最晚霜冻灾害日期对华南地区温度变化亦具有指示作用。其中霜冻灾害南界、冰冻灾害南界、降雪南界向南推进1个纬度, 华南11~2月平均气温分别约低0.23℃、0.41℃和0.40℃; 南岭两侧50km范围内的降雪日数每多(少)10天, 华南11~2月平均气温约低(高)0.22℃。受原始记载缺失及距今越近、记录数量越多等的影响, 1400~1500年间有记录的年份只有7年, 而1501~1950年间有记录的年份则有309年。1951年以来, 冰冻灾害和降雪南界从未到海南, 霜冻灾害南界到海南岛的年份有8年; 而1501~1950年间, 冰冻灾害南界虽也从未到达海南岛, 但降雪南界有5年、霜冻灾害南界有22年到达海南岛, 且整个区域的初(终)霜冻灾害平均日期也早(晚)于1951年以后。这为下一步重建这一地区更长时段的年分辨率温度序列提供了重要依据。     

Research Progress of Long-term Ocean Temperature Changes in the Southern Ocean

In the recent decades， a large amount of anthropogenic heat has been absorbed and stored in the Southern Ocean. Results from observations and climate models' simulations both show that the Southern Ocean displays large warming in the upper and subsurface ocean that maximizes at 45°~40°S. However， the underlying mechanisms and evolution processes of the Southern Ocean temperature changes remain unclear， leaving the Southern Ocean to be a hotspot of climate change studies in the recent years. The present study summarized the current progress in the observations and numerical modeling of long-term temperature changes in the Southern Ocean. The effects of changes in wind， surface heat flux， sea-ice and other factors on the ocean temperature changes were presented， along with the introduction to the role of oceanic mean circulation and eddies. The present study further proposed that a deepening of the understanding in the Southern Ocean temperature change may be achieved by investigating the fast and slow responses of the Southern Ocean to external radiative forcing， which are respectively associated with the fast adjustments of the ocean mixed-layer and the slow evolution of the deep ocean. Specifically， the striking and fast mixed-layer ocean warming north of 50°S is tightly related to the surface heat absorption over upwelling regions and wind-driven meridional heat transport， resulting in enhanced warming around 45°S. While in the slow response of the Southern Ocean temperature， the enhanced ocean warming shifts southward and downward， mainly associating with the heat transfer from oceanic eddies. The Southern Ocean temperature has pronounced climatic effects on many aspects， such as global energy balance， sea-level rise， ocean stratification changes， regional surface warming and atmospheric circulation changes. However， large model biases/deficiencies in simulating the present-day climatology and essential ocean dynamic processes last in generations of climate models， which are the main challenge in advancing our understanding in the mechanisms for the Southern Ocean climate changes. Therefore， to achieve reliable future projections of the Southern Ocean climate， substantial efforts will be needed to improve the model performances and physical understanding in the relative role of various processes in ocean temperature changes at different time scales.     

Ocean sea-ice modelling in the Southern Ocean around Indian Antarctic stations

An eddy-resolving coupled ocean sea-ice modelling is carried out in the Southern Ocean region (9\(^{\circ }\)–78\(^{\circ }\)E; 51\(^{\circ }\)–71\(^{\circ }\)S) using the MITgcm. The model domain incorporates the Indian Antarctic stations, Maitri (11.7\({^{\circ }}\)E; 70.7\({^{\circ }}\)S) and Bharati (76.1\({^{\circ }}\)E; 69.4\({^{\circ }}\)S). The realistic simulation of the surface variables, namely, sea surface temperature (SST), sea surface salinity (SSS), surface currents, sea ice concentration (SIC) and sea ice thickness (SIT) is presented for the period of 1997–2012. The horizontal resolution of the model varies between 6 and 10 km. The highest vertical resolution of 5 m is taken near the surface, which gradually increases with increasing depths. The seasonal variability of the SST, SSS, SIC and currents is compared with the available observations in the region of study. It is found that the SIC of the model domain is increasing at a rate of 0.09% per month (nearly 1% per year), whereas, the SIC near Maitri and Bharati regions is increasing at a rate of 0.14 and 0.03% per month, respectively. The variability of the drift of the sea-ice is also estimated over the period of simulation. It is also found that the sea ice volume of the region increases at the rate of 0.0004 \(\hbox {km}^{3}\) per month (nearly 0.005 \(\hbox {km}^{3}\) per year). Further, it is revealed that the accumulation of sea ice around Bharati station is more as compared to Maitri station.     

Sediment flux distribution in the Southern Brazil Basin during the late Quaternary: the role of deep-sea currents

Detailed sedimentological and stratigraphic analyses were carried out on seven Kullenberg cores collected across the Brazilian continental margin during the French cruises Byblos and Apsara III, in order to highlight the factors controlling the sediment flux distribution in the Southern Brazil Basin during the late Quaternary. On the continental slope and upper continental rise above 3000 m depth, sediment fluxes are important and highly variable (4·2–14·2 g cm^?2 10^?3 yr). The sediments show a pelagic or turbiditic character, depending on the width of the shelf and proximity of canyons. The material is characterized by high kaolinite contents, and originates from the coastal rivers draining the South American continent north of Rio de Janeiro. On the middle continental rise between 3000 and 4000 m depth, sediment fluxes are the lowest observed in the area (0·9 g cm^?2 10^?3 yr), because terrigenous input is trapped at shallower depths on the São Paulo Plateau. Pelagic settling is the dominant process. In the deep domains, below 4000 m depth, contouritic accumulations are developed on the path of the northwards moving Antarctic bottom water (AABW) currents. The deposits consist of fine-grained silty-clayey muds with very low carbonate contents. The sediment fluxes (1·45 g cm^?2 10^?3 yr) are higher than on the middle continental rise, as a consequence of fine-grained terrigenous supply derived from higher latitudes (Argentine Basin and Southern Ocean), and transported in the basin through the Vema Channel by the AABW currents. This material is characterized by high smectite and chlorite contents. These data reveal large sediment flux variations which are linked to distinct depth-related domains. Such a distribution is the consequence of the presence of two available sources of terrigenous sediments: (1) the Brazilian continental areas with a downslope material transport and a sediment distribution controlled by the morphology of the margin, and (2) the Argentine Basin with an alongslope material transport by deep-sea currents which dominate the sedimentation in the abyssal domains.     

Geochemical and mineralogical control of different genetic types of deep-sea nodules from the Pacific Ocean

Deep-sea nodules from the Northeast Pacific nodule belt and the Southeast Pacific (Sonne Basin), being formed in areas bordering the equatorial zone of high biological productivity, accumulate by two basically different growth processes: (A) early diagenetic growth by supply from pore water and (B) hydrogenetic growth by supply from near-bottom sea-water. These growth processes lead to different genetic types of nodules: early diagenetic type A, hydrogenetic type B, and mixed-type AB; a further type A_C, very rich in Mn, is being formed by increasing influence of early diagenesis. These types can clearly be distinguished by their shapes, surface textures, mineral constituents of oxide fraction, internal microstructures, and geochemistry. A genetical classification is being proposed on the basis of statistically computed interelement relationships. Todorokite, very poor in Fe, is the main Mn phase in the early diagenetic substance; -MnO₂ intimately intergrown with FeOOH · xH₂O is the main phase in the hydrogenetic substance. Consequently an important difference can be pointed out: the metal supply for the growth of the early diagenetic nodules is based on an ionic solution of Me²⁺ (e. g. Mn²⁺, Ni²⁺, Cu²⁺, Zn²⁺), whereas the supply for the hydrogenetic nodules is caused by transport of colloidal particles. Mobilization of Mn²⁺ and fractionation from Fe is controlled by the amount of decomposing organic matter in the "peneliquid" layer of the sediments. The main factor controlling the intensity of early diagenesis is the biological productivity in surface waters. The crucial "point of reversal" at a Mn/Fe ratio of about 5, obtained by hyperbolical regression of the analyses of nodules from the Southeast Pacific, represents best concentrations in Ni and Cu. Mn/Fe quotients greater than 5 cause a decrease of Ni and Cu content. Nodules from the Northeast Pacific nodule belt generally contain higher concentrations in Cu than nodules from the Southeast Pacific. This can be explained by an additional supply of Cu transported below CCD by siliceous plankton.     

Geochemistry of deep-sea sediments in two cores retrieved at the mouth of the Coatzacoalcos River delta,western Gulf of Mexico,Mexico

Coastal margins, especially the river-influenced coastal areas, are considered as active interfaces between the continental and oceanic environments, which have huge dispersal of detrital materials and heavy metal input. It is well determined that the fine-grained sediments are important reservoir for the accumulation of heavy metals. In this study, we analyzed the radiocarbon age, texture, organic matter, carbonate content, and geochemical compositions of two sediment cores (GM42 and GM44) retrieved in front of the Coatzacoalcos River mouth basin, southwestern Gulf of Mexico (～864 and 845 m water depth, respectively). Our objective was to infer the sedimentation rate, intensity of weathering, provenance, and influence of anthropogenic activities on heavy metal contamination in sediments. The radiocarbon-age measurements of mixed planktonic foraminifera for core GM44 reveals an age of 21,289 ± 136 cal. years B.P., which fall within the Late Glacial Maximum (LGM; 21000 ± 2000 years B.P). The calculated sedimentation rate for core GM42 (~0.013 cm/year) is lower than in core GM44 (0.022 cm/year), which is probably due to the variations in detrital sediment input and/or seafloor topography. The weathering indices such as chemical index of alteration (CIA), chemical index of weathering (CIW), and plagioclase index of alteration (PIA) suggested that the source area experienced low to moderate intensity of chemical weathering under warm to humid climatic conditions. The SiO₂/Al₂O₃, Al₂O₃/Na₂O, and K₂O/Al₂O₃ ratio values indicated moderate to high compositional maturity. The major and trace element concentrations suggested that the sediments were likely derived from intermediate source rocks. The heavy metal contents indicated that the sediments were not contaminated by the industrial waste disposals supplied by the Coatzacoalcos River. The redox proxy sensitive elements such as V, Cr, Cu, and Zn indicated an oxic depositional environment for the deep-sea sediment cores. The application of discrimination diagrams for the geochemistry data revealed a passive margin setting for the sediment cores. The compositional variations observed at the upper sections (<30 cm) between the two sediment cores revealed that the type of detrital sediments supplied by the Coatzacoalcos River to the deep sea area is not uniform, which is also revealed by the variation in sedimentation rate.     

Geochemistry of two sediment cores from the west coast of India

Copper, Pb, Zn, Ni, Co, Mn, AI, Ca, magnetic susceptibility and loss on ignition of sediments from two cores near Mangalore along the western continental shelf of India have been studied. The sediments have high Al and organic matter contents due to the high sedimentation rate and their proximity to river mouths. Down-core variations of elements indicate a decrease of lithogenous component during probably the past few centuries. While abundance of calcareous shells in some zones has lead to the dilution of most of the metals, it appears that Pb and Mn are associated with this phase. Copper, Zn and Fe are associated with organic matter and detrital particles, whereas Ni and Co are predominantly associated with the insoluble fraction. Oxides/hydroxides of Fe and Mn are absent because of the reducing conditions and the high terrigenous influx. Geochemically, Mn and Fe are present in different phases of sediments (in the insoluble fraction and organic matter respectively). The Fe content of one of the cores is positively correlated with magnetic susceptibility.     

Dysaerobic conditions during Heinrich events 4 and 5: Evidence from phosphorus distribution in a North Atlantic deep-sea core

Reactive phosphorus undergoes diagenetic transformation once transferred into marine sediments. The degree of regeneration and redistribution of phosphorus depends on early diagenetic and environmental conditions, which may be linked to larger scale phenomena, such as bottom water circulation, water column ventilation, and organic carbon flux. Phosphorus phases of the <50-μm-sized fraction of deep-sea sediments from core SU 90-09 (North Atlantic, 43°31′N, 30°24′W, 3375 m below sea level) have been analyzed using a sequential extraction technique (SEDEX method) to reconstruct phosphorus geochemistry during Heinrich events 4 and 5. Comparison with Holocene samples from the same site indicates that postdeposition diagenetic transformation has not affected phosphorus distribution in the deep part of the sediments. Total and reactive phosphorus average 0.40 ± 0.04 mg/g and 0.30 ± 0.05 mg/g, respectively, and are comparable to values found in analog deep-sea environments in the North Atlantic. Detrital phosphorus, the phase linked to igneous- and metamorphic-derived material, sharply increases during Heinrich events and covaries with the ice-rafted debris record, whereas authigenic and Fe-bound phosphorus phases, both influenced by redox conditions, decrease or even disappear. These findings suggest that during the deposition of Heinrich layers (HLs), environmental parameters hampered the precipitation of these phases. Large freshwater discharges in relation to iceberg surges may have provoked a temporary stratification of the water column. Accordingly, dysaerobic conditions in the sediments may have fostered the loss of dissolved phosphorus from the sediments to the water column, in a direct and rapid response to the changed conditions. Decreasing trends in organic matter elemental ratios (total organic carbon/organic phosphorus) and Rock-Eval oxygen index values, along with the presence of partly authigenic dolomite and ankerite within HLs, also support this assumption.