Radiocarbon Geochemistry of Modern and Ancient Arctic Lake Systems, Baffin Island, Canada

The accuracy of Arctic lake chronologies has been assessed by measuring the¹⁴C activities of modern carbon sources and applying these isotopic mass balances to dating fossil lake materials. Small (<1 km²) shallow (<25 m) Arctic lakes with watersheds <12 km²have soil and peat stratigraphic sections with¹⁴C activities ranging from 98 to 51% Modern. The¹⁴C activity of particulate organic carbon, dissolved organic carbon, and dissolved inorganic carbon from lake and stream waters ranges from 121 to 95% Modern. The sediment–water interface of the studied lakes shows consistent¹⁴C ages of ∼1000¹⁴C yr, although the¹⁴C activity of living aquatic vegetation is 115% Modern. Radiocarbon measurements of components of the lacustrine carbon pool imply that the ∼1000¹⁴C yr age of the sediment–water interface results from deposition of¹⁴C-depleted organic matter derived from the watershed.     

Paleoindian environmental change and landscape response in Barger Gulch,Middle Park,Colorado

Middle Park, a high‐altitude basin in the Southern Rocky Mountains of north‐central Colorado, contains at least 59 known Paleoindian localities. At Barger Gulch Locality B, an extensive Folsom assemblage (˜10,500 ¹⁴C yr B.P.) occurs within a buried soil. Radiocarbon ages of charcoal and soil organic matter, as well as stratigraphic positions of artifacts, indicate the soil is a composite of a truncated, latest‐Pleistocene soil and a younger mollic epipedon formed between ˜6000 and 5200 ¹⁴C yr B.P. and partially welded onto the older soil following erosion and truncation. Radiocarbon ages from an alluvial terrace adjacent to the excavation area indicate that erosion followed by aggradation occurred between ˜10,200 and 9700 ¹⁴C yr B.P., and that the erosion is likely related to truncation of the latest‐Pleistocene soil. Erosion along the main axis of Barger Gulch occurring between ˜10,000 and 9700 ¹⁴C yr B.P. was followed by rapid aggradation between ˜9700 and 9550 ¹⁴C yr B.P., which, along with the erosion at Locality B, coincides with the abrupt onset of monsoonal precipitation following cooling in the region ˜11,000–10,000 ¹⁴C yr B.P. during the Younger Dryas oscillation. Buried soils dated between ˜9500 and 8000 ¹⁴C yr B.P. indicate relative landscape stability and soil formation throughout Middle Park. Morphological characteristics displayed by early Holocene soils suggest pedogenesis under parkland vegetation in areas currently characterized by sagebrush steppe. The expansion of forest cover into lower elevations during the early Holocene may have resulted in lower productivity in regards to mammalian fauna, and may partly explain the abundance of early Paleoindian sites (˜11,000–10,000 ¹⁴C yr B.P., 76%) relative to late Paleoindian sites (˜10,000–8000 ¹⁴C yr B.P., 24%) documented in Middle Park. © 2005 Wiley Periodicals, Inc.     

Aspartic acid racemization in Late Wisconsin Lake Ontario sediments

We have determined aspartic acid racemization and [¹⁴C] ages in sediment from a 17-m piston core recovered from Lake Ontario. This core represents a depositional record extending back into the Late Wisconsin. Total organic radiocarbon ages of the glaciolacustrine lower section of the core are older than the true depositional age. Hence we suggest that these ages do not represent time elapsed since deposition but rather may be up to several thousand years too old due to mixing with ¹⁴C-depleted organic matter and possibly also d-amino acids contained by the glacier and deposited as the glacier receded. Further studies are suggested that might test this hypothesis as well as establish the general applicability of racemization dating to clay sediments.     

Comparison of radiocarbon and uranium-series dating methods as applied to marine apatite

A comparison of uranium-series geochronology and radiocarbon dating, the first systematic test of its kind applied to authigenic marine apatite, has been made for nine seafloor phosphate nodules sampled from the Peruvian and Chilean shelves. Radiocarbon ages agree well with the ²³⁰Th ages for all six samples with ²³⁰Th ages younger than the practical dating limit of the ¹⁴C technique for marine samples. Two of three “old” samples contain no significant ¹⁴C activities, as expected. However, one sample with a high concentration of phosphate pellets contains more than 30% of the modern ¹⁴C activity. The discrepancy between the ¹⁴C and U-series age for this sample is thought to be due to post-depositional adsorption of the particle-reactive elements thorium and protactinium onto pellet surfaces.     

Luminescence dating of lacustrine sediments from Tangra Yumco (southern Tibetan Plateau) using post‐IR IRSL signals from polymineral grains

Radiocarbon dating of bulk organic matter is the most commonly used method for establishing chronologies of lake sediments for palaeoclimate reconstructions on the Tibetan Plateau. However, this method is likely to be problematic because the dated material often suffers from old carbon contamination. Recently, advances in luminescence‐based chronological techniques have provided new options for dating lacustrine sediments. In the current study, we tested for the first time the applicability of a new post‐IR IRSL (pIRIR) measurement protocol for dating fine‐grained polymineral material from a deep‐lake sediment core from the central part of Tangra Yumco, on the southern Tibetan Plateau. Our results show that: (i) radioactive disequilibria in the uranium decay chain were observed in the studied lake sediments, and thus taken into account for dose rate calculation by using a dynamic modelling approach; (ii) the suitability and robustness of the pIRIR protocol measured at 150°C (pIRIR₁₅₀) for our samples are confirmed by a set of luminescence characteristic tests as well as the agreement with an independent age control; (iii) turbidite deposition partly caused an insufficient resetting of luminescence signals and thus apparent overestimation in luminescence dating; (iv) compared with the luminescence‐based age‐depth model, the ¹⁴C ages of bulk organic matter from the studied core generally yielded an age difference of ～2 ka, which is attributed to hardwater reservoir effects in Tangra Yumco. This study highlights the need for multi‐dating approaches of lake sedimentary archives on the Tibetan Plateau.     

Isolation and compound specific radiocarbon dating of terrigenous branched glycerol dialkyl glycerol tetraethers (brGDGTs)

The relative abundances and distributions of branched glycerol dialkyl glycerol tetraethers (brGDGTs) in marine and lacustrine sediments have been recently used as paleo-environmental proxies for the input of soil organic matter and past continental temperatures, respectively. The putative main source of brGDGTs is soil bacteria. Therefore, these biomarkers are also part of the soil organic matter stock that accumulates in the terrigenous environment. Here we present a method for isolating and purifying these compounds from lake sediments, soils and peat for radiocarbon (¹⁴C) measurement using a combination of normal and reversed phase high performance liquid chromatography, allowing complete separation of co-eluting compounds. Radiocarbon analysis was performed on an accelerator mass spectrometer system equipped with a gas ion source. We assessed the accuracy and the necessary corrections with modern and radiocarbon-dead procedural blanks. Blank assessment revealed that the reverse phase step introduced significant amounts of extraneous carbon, while the normal phase step did not. We show that with our method and instrumentation it is possible to obtain accurate and precise ¹⁴C analyses of brGDGTs with sample amounts as small as 10 μg C. Finally we demonstrate the application of the measurement of ¹⁴C concentrations of several brGDGTs isolated from lake sediments and peat soils.     

Pyrolysis-combustion C dating of soil organic matter

Radiocarbon (¹⁴C) dating of total soil organic matter (SOM) often yields results inconsistent with the stratigraphic sequence. The onerous chemical extractions for SOM fractions do not always produce satisfactory ¹⁴C dates. In an effort to develop an alternative method, the pyrolysis-combustion technique was investigated to partition SOM into pyrolysis volatile (Py-V) and pyrolysis residue (Py-R) fractions. The Py-V fractions obtained from a thick glacigenic loess succession in Illinois yielded ¹⁴C dates much younger but more reasonable than the counterpart Py-R fractions for the soil residence time. Carbon isotopic composition (δ¹³C) was heavier in the Py-V fractions, suggesting a greater abundance of carbohydrate- and protein-related constituents, and δ¹³C was lighter in the Py-R fractions, suggesting more lignin- and lipid-related constituents. The combination of ¹⁴C dates and δ¹³C values indicates that the Py-V fractions are less biodegradation resistant and the Py-R fractions are more biodegradation resistant. The pyrolysis-combustion method provides a less cumbersome approach for ¹⁴C dating of SOM fractions. With further study, this method may become a useful tool for analyzing unlithified terrestrial sediments when macrofossils are absent.     

An assessment and comparison of 230Th and AMS 14C ages for lacustrine sediments from Qarhan Salt Lake area in arid western China

The age framework of Qarhan Salt Lake in arid western China is still controversial due in part to (1) age discrepancy between conventional ¹⁴C and ²³⁰Th dating results, and (2) no AMS ¹⁴C ages of organic carbon from drilling cores in Qarhan Salt Lake were reported until now. In order to discuss these chronological problems, upper 54.50 m lacustrine sediments from a drilling core (ISL1A) recovered from Qarhan Salt Lake were dated based on ²³⁰Th and AMS ¹⁴C dating techniques. Results show that (1) AMS ¹⁴C ages of total organic carbon (TOC) from 4.65 to 30.29 m are almost in stratigraphic order and consistent with ²³⁰Th ages of halite in the corresponding layers; (2) AMS ¹⁴C ages of TOC from 30.29 to 54.50 m are younger with increasing depth. This phenomenon was also found in Shell Bar in the study area, suggesting that AMS ¹⁴C ages from upper 30.29 m are more reliable while those from lower 24.21 m in ISL1A may be underestimated; (3) ²³⁰Th ages of halite from lower 24 m lacustrine sediments are obviously older than AMS ¹⁴C ages of TOC in the corresponding layers, which results into different age framework of salt lake sediments in Qarhan Salt Lake; (4) if extrapolating these reliable AMS ¹⁴C ages in ISL1A, similar age framework with ²³⁰Th ages in this core confirms that ²³⁰Th ages are much close to the true ages of these sediments, which suggests that the forming timing of the bottom salt layer is ~50 ka.     

Late Holocene radiocarbon and aspartic acid racemization dating of deep-sea octocorals

Primnoa resedaeformis is a deep-sea gorgonian coral with a two-part skeleton of calcite and gorgonin (a fibrillar protein), potentially containing long-term records of valuable paleo-environmental information. For various reasons, both radiocarbon and U/Th dating of these corals is problematic over the last few centuries. This paper explores aspartic acid racemization dating of the gorgonin fraction in modern and fossil specimens collected from the NW Atlantic Ocean. Radiocarbon dating of the fossil specimen indicates a lifespan of 700 ± 100 years, the longest yet documented for any octocoral. Gorgonin amino acid compositions were identical in the fossil and modern specimens, indicating resistance to organic diagenesis. Similar to bone collagen, the fibrillar protein of gorgonin may impose conformational constraints on the racemization of Asp at low temperatures. The rate of racemization of aspartic acid (d/l-Asp) was similar to previously published results from an 1800 year old anemone (Gerardia). The age equation was: age (years BP 2000 AD) = [(d/l − 0.020 (±.002))/.0011 (±.0001)]² (r² = 0.97, p < .001). The error in an age estimate calculated by d/l-Asp was marginally better than that for ¹⁴C dating over the most recent 50-200 years, although the dating error may be improved by inclusion of more samples over a broader time range. These results suggest that d/l-Asp dating may be useful in augmenting ¹⁴C dating in cases where ¹⁴C calibrations yield two or more intercept ages, or in screening samples for further ¹⁴C or U/Th dating.     

Durations of soil formation and soil development indices in a Holocene Mediterranean floodplain

Holocene and late Pleistocene alluvial sequences of the mid-Medjerda floodplain (Northern Tunisia) reveal three types of soils with gradual transitions: Fluvisols (Calcaric), Cambisols (Calcaric) and Calcic Luvisols (Chromic). Stratigraphic cross-correlations, palaeomagnetic secular variation, and ¹⁴C and IRSL dating enable detailed information about ages and durations of soil formation in the floodplain.Weakly developed Fluvisols (Calcaric) commonly reveal late Holocene ages with soil formation durations lasting between 100 and 300 years. Hence, Holocene soil formation is detectable in the exposures from durations of around 100 years onwards. Cambisols show predominantly mid-Holocene ages. The durations of soil formation are between 800 and 5000 years. Calcic Luvisols (Chromic) feature late Pleistocene ages, with durations of soil formation between 10,000 and 40,000 years.Profile Development Indices were computed from simple field parameters like soil structure, soil colour, horizon thickness and leaching features. The derived development indices show good correlation (R² = 0.804) with the calculated durations of soil formation. Field parameters are well suited for a quantitative development index of Holocene soil formation, even if very weak developed soils are predominant. The study shows that maturity stages of Holocene alluvial soils in a homogeneous Mediterranean environment are predominantly driven by soil formation duration.     

不同植物凋落物对土壤有机碳淋失的影响及岩溶效应

以低含量有机碳的岩溶土壤（ＳＯＣ, ０．８９％）为媒介,每１５０ｇ上添加松针、梧桐叶粉各 ７．５ｇ,１５ｇ,４个试验土柱号分别为ＳＣＣ３,ＳＣＣ５;ＳＢＣ３,ＳＢＣ５,接种岩溶土壤微生物群落后,于 恒温室内进行培养淋溶实验。结果表明,土壤淋溶液的电导值受土壤有机质含量多少及性质 的影响。土壤水溶性有机碳（ＤＯＣ）淋失总量ＳＣＣ３为５４０．７ｍｇ,ＳＣＣ５为１５２２．９ｍｇ;ＳＢＣ３为 ３８３．２ｍｇ,ＳＢＣ５为５６３．５ｍｇ。同时,土壤环境中Ｃａ^２＋的释放总量ＳＣＣ３为１４５．７ｍｇ,ＳＣＣ５为 ２８８．７ｍｇ; ＳＢＣ３为１７０．０ｍｇ,ＳＢＣ５为１６７．９ｍｇ。两者呈正相关,相关系数ｒ^２＝０．８５。下伏碳酸 盐岩的溶蚀量排序为ＳＣＣ５＞ＳＢＣ３＞ＳＣＣ３＞ＳＢＣ５,表明两种不同植物凋落物经微生物分 解,产生不同质和量的ＤＯＣ,并导致土壤环境中 Ｃａ^２＋释放的差异和不同的岩溶效应。从而初步揭示不同有机碳分解导致ＤＯＣ淋失的差异性,以及ＤＯＣ对岩溶动力系统的驱动。这与Ａ．Ｈｅｙｅｓ和Ｔ．Ｒ．Ｍｏｏｒｅ的研究结果一致。     

Age assignment of a diatomaceous ooze deposited in the western Amundsen Sea Embayment after the Last Glacial Maximum

Reliable dating of glaciomarine sediments deposited on the Antarctic shelf since the Last Glacial Maximum (LGM) is challenging because of the rarity of calcareous (micro‐) fossils and the recycling of fossil organic matter. Consequently, radiocarbon (¹⁴C) ages of the acid‐insoluble organic fraction (AIO) of the sediments bear uncertainties that are difficult to quantify. Here we present the results of three different methods to date a sedimentary unit consisting of diatomaceous ooze and diatomaceous mud that was deposited following the last deglaciation at five core sites on the inner shelf in the western Amundsen Sea (West Antarctica). In three cores conventional ¹⁴C dating of the AIO in bulk samples yielded age reversals down‐core, but at all sites the AIO ¹⁴C ages obtained from diatomaceous ooze within the diatom‐rich unit yielded similar uncorrected ¹⁴C ages between 13 517 ± 56 and 11 543 ± 47 years before present (a BP). Correction of these ages by subtracting the core‐top ages, which probably reflect present‐day deposition (as indicated by ²¹⁰Pb dating of the sediment surface at one core site), yielded ages between ca. 10 500 and 8400 cal. a BP. Correction of the AIO ages of the diatomaceous ooze by only subtracting the marine reservoir effect (MRE) of 1300 a indicated deposition of the diatom‐rich sediments between 14 100 and 11 900 cal. a BP. Most of these ages are consistent with age constraints between 13.0 and 8.0 ka for the diatom‐rich unit, which we obtained by correlating the relative palaeomagnetic intensity (RPI) records of three of the sediment cores with global and regional reference curves. As a third dating technique we applied conventional radiocarbon dating of the AIO included in acid‐cleaned diatom hard parts extracted from the diatomaceous ooze. This method yielded uncorrected ¹⁴C ages of only 5111 ± 38 and 5106 ± 38 a BP, respectively. We reject these young ages, because they are likely to be overprinted by the adsorption of modern atmospheric carbon dioxide onto the surfaces of the diatom hard parts prior to sample graphitisation and combustion for ¹⁴C dating. The deposition of the diatom‐rich unit in the western Amundsen Sea suggests deglaciation of the inner shelf before ca. 13 ka BP. The deposition of diatomaceous oozes elsewhere on the Antarctic shelf around the same time, however, seems to be coincidental rather than directly related. Copyright © 2009 John Wiley & Sons, Ltd.     

柴达木盆地察尔汗贝壳堤剖面年代学研究*

利用常规¹⁴ C、加速器(AMS)和²³⁰ Th等测年方法,对位于青藏高原东北部柴达木盆地察尔汗古湖贝壳堤剖面化石贝壳、湖泊沉积(包括现代沉积)样品有机质(包括碱性残留和胡敏酸组分)、碳酸盐(CaCO₃)和剖面顶部石盐(NaCl)晶体在不同实验室进行了系统的实验研究,探讨了不同样品测年结果的可靠性和精确性,并通过与已有研究结果的对比,对柴达木盆地贝壳堤剖面记录的察尔汗古湖高湖面演化历史进行了探讨,得出察尔汗古湖高湖面形成于约39.7~17.5 ¹⁴ C kaB.P. ; 研究证明柴达木盆地沉积物中有机质含量很低,且主要来源于菌藻类等低等藻类和微生物,受老碳和溶解物质影响强烈,因此必须经过多种方法综合测年才能比较可靠地确定所测样品的形成年代; 测年结果的不确定性不仅来源于样品自身元素(同位素)的特性和沉积过程中再沉积作用的影响,也可能源自于放射性¹⁴ C产生过程的不稳定性; 对比发现贝壳化石老于同层位有机质¹⁴ C年龄15~18ka,这可能与柴达木盆地所处的特殊地理位置和巨大的古湖泊面积及水体有关; 同时,不平衡铀系测年在确定开放体系矿物晶体年代应用中的可靠性尚须进一步深入分析,其机理也还需要探讨和实验研究。     

Comparison of thermoluminescence and radiocarbon age-determinations from late-pleistocene alluvial deposits near Sydney, Australia

Radiocarbon and thermoluminescence (TL) age-determinations have been obtained for a large Pleistocene alluvial terrace on the Nepean River near Sydney, New South Wales, Australia. The deposit was laid down by a braided river system prior to the last glacial maximum. Six thoroughly pretreated samples of charcoal and degraded wood buried within gravels at the base of the terrace yielded apparent ¹⁴C ages of 37,000–42,000 yr B.P. These compare favorably with four TL determinations that gave apparent ages of 41,000–47,000 yr B.P. for the same deposit. Adjustment of the ¹⁴C ages to take geomagnetic effects into account further improves the correlation between these two independent dating techniques. In addition, ¹⁴C and TL correctly identified a reworked portion of the fine-grained alluvial overburden as being substantially younger than the main body of the terrace. These results augur well for the usefulness of TL age determinations of certain alluvial deposits.     

Characterization of soil organic carbon in drained thaw-lake basins of Arctic Alaska using NMR and FTIR photoacoustic spectroscopy

Arctic soils contain a large fraction of Earth’s stored carbon. Temperature increases in the Arctic may enhance decomposition of this stored carbon, shifting the role of Arctic soils from a net sink to a new source of atmospheric CO₂. Predicting the impact of Arctic warming on soil carbon reserves requires knowledge of the composition of the stored organic matter. Here, we employ solid state ¹³C nuclear magnetic resonance (NMR) spectroscopy and Fourier transform infrared-photoacoustic spectroscopy (FTIR-PAS) to investigate the chemical composition of soil organic matter collected from drained thaw-lake basins ranging in age from 0 to 5500 years before present (y BP). The ¹³C NMR and FTIR-PAS data were largely congruent. Surface horizons contain relatively large amounts of O-alkyl carbon, suggesting that the soil organic matter is rich in labile constituents. Soil organic matter decreases with depth with the relative amounts of O-alkyl carbon decreasing and aromatic carbon increasing. These data indicate that lower horizons are in a more advanced stage of decomposition than upper horizons. Nonetheless, a substantial fraction of carbon in lower horizons, even for ancient thaw-lake basins (2000-5500 y BP), is present as O-alkyl carbon reflecting the preservation of intrinsically labile organic matter constituents. Climate change-induced increases in the depth of the soil active layer are expected to accelerate the depletion of this carbon.     

Chronology of the shell bar section and a discussion on the ages of the Late Pleistocene lacustrine deposits in the paleolake Qarhan, Qaidam basin

Shell bar, composed of abundant fossil shells of Corbicula fluminea müller and Corbicula largillierti philippi and located at the southeastern end (36°30′N, 96°12′E) of the paleolake Quarhan, is one of the most prominent features in the Qaidam basin. It is the highest site where such species of fossil shells have been found in the Late Pleistocene age. A 2.6-m-thick fresh profile was manually excavated to determine the formation ages and the scope of the high paleolake levels. Accelerator mass spectrometry (AMS), conventional radiocarbon dating, and sector inductively coupled plasma-mass spectroscopy ²³⁰Th methods were used to investigate the reliability and accuracy of dating results in different laboratories. Ages of various components (e.g. acid residual and acid soluble fraction of the organic matter) from the same sample were determined. Age differences of a variety of materials (e.g. organic matter, fossil shells, and salt crystals), and age differences at the same sampling position were evaluated. Dating shows that the AMS ages given by alkali residue and acid soluble fractions vary greatly, e.g., from 124.5 cm upwards. The dates given by the acid soluble fraction were normal. Down the column, the dates show a reverse pattern and those of alkali residues, especially the lower part of the section, show an unstable pattern. These imply that organic matter had been influenced by two separate processes. One possible explanation is that the alkali residual fraction most likely had been contaminated by dead carbon-bearing reworked material because the study section is located near the edge of the paleolake and could have been easily influenced by old eroded deposits. Another is that the acid soluble fraction could have been contaminated by upward-flowing groundwater containing soluble organic matter. The amount of this soluble organic material should be very small because the strongest age reversal is in the lower part, where the TOC content remains low, meaning that the change of soluble organic fraction alters neither TOC content nor the δ ¹³C dramatically. It is concluded that a uniform mega-paleolake developed in the Qaidam basin in the northeastern Tibetan plateau between 39.7 and 17.5 ¹⁴C kaB.P. During the period when the high paleolake level of Qarhan was formed, the huge paleolake covered a vast area with dramatic lake level fluctuations. It is found that the ages determined may be influenced by either radioactive ¹⁴C variations of repeated deposition or content variations in the atmosphere. The ages given by fossil shells are 15–18 ka older than those given by organic matter. The large differences between the ages of fossil shells and organic matter might have resulted from the large water areas and huge water volume and the special location of the high-elevation Qaidam basin. This study also shows the reliability and accuracy of the ²³⁰Th dating method on the salt crystals but further study is needed to determine whether this method could be applied to the study area. __________ Translated from Quaternary Sciences, 2007, 27(4): 511–521 [译自: 第四纪研究]     

A new correction model for C ages in aquifers with complex geochemistry – Application to the Neogene Aquifer,Belgium

The objective of this paper is to build a general correction model that takes into account all the different radiocarbon-dilution reactions and resolving the processes that are geochemically “aging” the groundwater in the Neogene Aquifer. For this, δ¹³C and radiocarbon in groundwater are investigated with their relationship to other chemical components in groundwater. The δ¹³C values in the Neogene Aquifer are influenced by various geochemical reactions like calcite dissolution, oxidation of organic matter and methanogenesis. Calcite dissolution and CH₄ production increase δ¹³C while the oxidation of organic matter decreases δ¹³C in the groundwater. The reactions that modify δ¹³C also influence the ¹⁴C activity. Due to the complex geochemical environment, existing correction models are not applicable to this situation. A correction model for initial ¹⁴C activity is formulated in which the different C sources that influence ¹⁴C activity are taken into account. It is observed that recent dissolved organic matter plays an important role in redox reactions. The corrected ¹⁴C ages lie between −0.792 and 6.425 ka representing the maximum age. If a part of the organic matter that oxidises is fossil, the determined age will represent an overestimated age.     

南海诸岛土壤的形成和年龄

本文在阐明南海珊瑚诸岛土壤的生物集聚过程、积盐和脱盐过程,以及有关的土壤诊断特性基础上,进一步介绍了那里的磷质石灰土和滨海盐土两大土壤类型,它们的空间分布及其与岛屿结构、地形、植被和鸟类活动之间的关系。最后,作者针对南海珊瑚岛土壤的特征,运用(１４)Ｃ方法探讨了它的年龄及其与土壤形成、发育之间的相互关系。     

Recycling of Organic Matter in the Sediments of Santa Monica Basin,California Borderland

Geochemical and isotopic data for the uppermost 1.2 m of the sediments of the central Santa Monica Basin plain were examined to better understand organic matter deposition and recycling at this site. Isotopic signatures (Δ¹⁴C and δ¹³C) of methane (CH₄) and dissolved inorganic carbon (DIC) indicate the occurrence of anaerobic oxidation of CH₄ that is fueled by CH₄ supplied from a relict reservoir that is decoupled from local organic carbon (C_org) degradation and methanogenesis. This finding was corroborated by a flux budget of pore-water solutes across the basal horizon of the profile. Together these results provide a plausible explanation for the anomalously low ratio between alkalinity production and sulfate consumption reported for these sediments over two decades ago. Shifts in Δ¹⁴C and δ¹³C signatures of C_org have previously been reported across the 20-cm depth horizon for this site and attributed to a transition from oxic to anoxic bottom water that occurred ~350 years BP. However, we show that this horizon also coincides with a boundary between the base of a hemipelagic mud section and the top of a turbidite interval, complicating the interpretation of organic geochemical data across this boundary. Radiocarbon signatures of DIC diffusing upward into surface sediments indicate that remineralization at depth is supported by relatively ¹⁴C-enriched C_org within the sedimentary matrix. While the exact nature of this C_org is unclear, possible sources are hemipelagic mud sections that were buried rapidly under thick turbidites, and ¹⁴C-rich moieties dispersed within C_org-poor turbidite sections.