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1.
We investigated the characteristics of the alkenones produced by a bloom of Emiliania huxleyi in the eastern Bering Sea in 2000. Alkenones were detected in surface waters between 57°N and 63°N, where phosphate concentrations were low and the ammonium/nitrate ratio was high. The total alkenone content (C37:2, C37:3, and C37:4) ranged from 22.0 to 349 μg g−1 in suspended particles and from 0.109 to 1.42 μg g−1 in surface sediments. This suggests that a large proportion of the particulate alkenones synthesized in the surface water rapidly degraded within the water column and/or at the water-sediment interface of the Bering Shelf. The change in the stable carbon isotopic composition (δ13C) of C37:3 alkenone could not be explained only by variation in [CO2(aq)] in the surface water but also depended on the growth rate of E. huxleyi. The alkenone unsaturation index (UK′37) was converted into an alkenone “temperature” with three equations [Prahl et al 1988], [Sikes et al 1997] and [Müller et al 1998]; Sikes et al.’s (1997) equation gave the best correlation with the observed sea surface temperature (SST) in the eastern Bering Sea. However, some temperatures estimated by Sikes et al.’s (1997) equation from the UK′37 varied from the observed SST, possibly because of the rapidly changing rate of alkenone synthesis in the logarithmic growth stage or the low rate of alkenone synthesis when nutrients were limiting. Temperatures estimated from UK′37 in the surface sediments (6.8-8.2°C) matched the observed SST in September (7-8°C) but differed from the annual average SST of 4 to 5°C, suggesting that most of the alkenone in the eastern Bering Sea was synthesized during limited periods, for instance, in September. The relative amounts of C37:4 alkenone as proportions of the total alkenones (referred to as C37:4%) were high, ranging from 18.3 to 41.4%. Low-salinity water (<32 psu) within the study area would have contributed to the high C37:4% because a negative linear relationship between C37:4% and salinity was found in this study.  相似文献   

2.
Alkenone unsaturation ratios of sedimentary lipids are used as a geochemical proxy for sea surface temperatures, and interest is growing in their potential as indicators of different water masses and possibly of salinity. We analyzed the abundance of unsaturated C37 to C38 ketones in lipid extracts of 57 surface sediment (0-1 cm) samples along a salinity gradient from 8 to 33 psu in the transition from the Skagerrak to the Baltic Sea (NW Europe). In addition to surface sediments, we analyzed alkenones in suspended particulate matter at 13 stations—over a gradient in salinity from 25 to 33 psu—during a bloom of the coccolithophore Emiliania huxleyi. Alkenones were detected in all samples (suspended matter and sediment) with variable contributions of the tetra-unsaturated C37 alkenone compound (%C37:4; range from 2 to 10% of total C37 alkenone content). Comparing the alkenone unsaturation index (U37K′) and %C37:4 data to climatological sea surface temperature and sea surface salinity data sets revealed that SST estimated from U37K′ of saline end members (samples from the Skagerrak) is in the general range of modern SST during bloom periods of haptophytes. At salinities below ∼30 psu %C37:4 increases to above 5% and the unsaturation ratios cease to be related to climatological annual or seasonal sea surface temperatures. On the other hand, the %C37:4 appears to be inversely and significantly correlated to salinity: Highest C37:4 proportions in the inner Baltic Sea are caused by an unidentified organism, but in the transition area at salinities down to 10 psu, the producer apparently is E. huxleyi. The suspended matter data together with those from the water column support the hypothesis of changing biosynthesis of alkenones under salt stress by the coccolithophore E. huxleyi, but constrain the maximum of %C37:4 attributable to salt stress to 10% of all C37 alkenones.  相似文献   

3.
The alkenone unsaturation index UK′37 has been applied to reconstruct past temperature changes in both marine and lacustrine systems. However, few studies have addressed whether the relative abundance of the C37:4 alkenone to the total C37 production (%C37:4) can reflect surface salinity changes in lacustrine systems. Here we present long-chain C37 alkenone distribution patterns in surface sediments from Lake Qinghai, China. Surface sediments were sampled over a large range of surface salinity changes (1.7-25 g/l) within Lake Qinghai and its surrounding lakes, while temperature differences at these sampling locations should be relatively small. We have found that %C37:4 varies from 15% to 49% as surface salinity decreases. We tentatively describe this %C37:4-salinity link with a general linear regression: %C37:4 = 53.4 (±7.8) − 1.73 (±0.45) × S (n = 28, r2 = 0.62), although step-wise %C37:4 changes in response to salinity variation may exist. UK′37 values vary between 0.10 and 0.16 at these sites and the inferred range of lake water temperature changes is ∼2-3 °C, suggesting that UK′37 largely reflects temperature signal across a large salinity range, consistent with previous findings that UK′37 can indicate temperature changes over a large diversity of environmental settings. We have also found that UK′37 values are correlated with salinity changes (r2 = 0.4), and thus cannot exclude potential temperature effect on %C37:4 and salinity effect on UK′37 in this study. However, even extreme estimates of temperature differences within the lake are still unable to explain the observed %C37:4 changes. We therefore suggest that %C37:4 could be used to infer past lake salinity changes at a regional scale.  相似文献   

4.
We compare alkenone unsaturation ratios measured on recent sediments from the Indian Ocean (20°N–45°S) with modern sea oceanographic parameters. For each of the core sites we estimated average seasonal cycles of sea surface temperature (SST) and salinity, which we then weighted with the seasonal productivity cycle derived from chlorophyll satellite imagery. The unsaturation index (U37K′) ranges from 0.2 to 1 and correlates with water temperature but not with salinity. TheU37K′versus SST relationship for Indian Ocean sediments (U37K′= 0.033 SST + 0.05) is similar to what has been observed for core tops from the Pacific and Atlantic oceans and the Black Sea. A global compilation for core tops givesU37K′= 0.031 T + 0.084 (R= 0.98), which is close to a previously reported calibration based on particulate organic matter from the water column. For temperatures between 24° and 29°C, however, the slope seems to decrease to about 0.02U37K′unit/°C. For Indian Ocean core tops, the ratios of total C37alkenones/total C38alkenones and the slope of theU37K′-SST relationship are similar to those previously observed for cultures ofEmiliania huxleyibut different from those previously published forGephyrocapsa oceanica.EitherE. huxleyiis a major producer of alkenones in the Indian Ocean or strains ofG. oceanicaliving in the northern Indian Ocean behave differently from the one cultured. In contrast with coccolithophorid assemblages, the ratios of C37alkenones to total C38alkenones lack clear geographic pattern in the Indian Ocean.  相似文献   

5.
Long chain alkenones (LCA) are temperature-sensitive lipids with great potential for quantitative reconstruction of past continental climate. We conducted the first survey for alkenone biomarkers from 55 different lakes in the Northern Great Plains and Nebraska Sand Hills of the United States. Among those surveyed, we found 13 lakes that contain LCAs in the surface sediments. The highest concentrations of alkenones in sediments are found in cold (mean annual air temperature ∼11 °C versus 17 °C in our warmest sites), brackish to mesosaline (salinity = 8.5-9.7 g/L), and alkaline (pH = 8.4-9.0) lakes with high concentrations of sodium and sulfate. The dynamics of stratification and nutrient availability also appear to play a role in LCA abundance, as early spring mixing promotes a bloom of alkenone-producing haptophytes. Four of the alkenone-containing sites contain the C37:4 alkenone; however, we discovered an unprecedented lacustrine alkenone distribution in a cluster of lakes, with a total absence of C37:4 alkenone. We attribute this unusual composition to a different haptophyte species and show that the sulfate:carbonate ratio may control the occurrence of these two distinct populations. We created a new in-situ temperature calibration for lacustrine sites that contain C37:4 using a water-column calibration from Lake George, ND and show that is linearly correlated to lake water temperature (R2 = 0.74), but is not. A number of lakes contain an unidentified compound series that elutes close to the LCAs, highlighting the importance of routine GC-MS examination prior to using lacustrine LCAs for paleotemperature reconstructions.  相似文献   

6.
The successful reconstruction of sea surface temperatures using alkenone paleothermometry (U37k′) has relied on the premise that there is no significant differential degradation of alkenones with different states of unsaturation during diagenetic processes. To test this assumption, we conducted a comparative study of contemporary sediments in oxic and anoxic bottom waters from the Santa Monica Basin, offshore California. Long-chain alkenones were quantified and sea surface temperature were calculated using the calibrated U37k′–T relationship of Prahl et al. (1988). Our results show that temperature record from the oxic sediments is higher by as much as 4°C compared to those from time-equivalent anoxic sediments as a result of differential degradation of long-chain unsaturated alkenones and bioturbation mixing in the oxic sediments. The differential degradation of C37:3 vs. C37:2 alone could account for up to 2.5°C difference between these two records. This finding has significant implication in the interpretation of paleo–sea surface temperature data using alkenone paleothermometry.  相似文献   

7.
The large (∼20‰) hydrogen isotopic gradient in surface waters of the northwest Atlantic Ocean is exploited to track changes in the source of alkenones to the Bermuda Rise sediment drift. Cultures of the predominant alkenone-producing coccolithophorid, E. huxleyi, were grown in deuterium-enriched seawater and shown to possess alkenones with a D/H ratio that closely tracked the water D/H ratio (r2 = 0.999, n = 5 isotopic enrichments) with a fractionation factor (α) between 0.732 and 0.775. A hydrogen isotopic depletion of -193 ± 3‰ (n = 9) was measured in alkenones from suspended particles relative to seawater in the subpolar and subtropical northwest Atlantic Ocean. This value was used to calculate the water δD values in which alkenones from Bermuda Rise sediment were synthesized, and by extension, the water mass in which they were produced. Applying this technique we find that 60% to 100% of the alkenones in late Holocene Bermuda Rise sediment were produced in deuterium-depleted subpolar water to the northwest of the drift. To reconcile values of the alkenone unsaturation ratio (Uk′37), a widely used proxy for sea surface temperature, with the δD values of alkenones in late Holocene sediments from the Bermuda Rise at least three sources of sediment must be invoked: a cold, very isotopically depleted source, almost certain to be the Scotian Margin; a warm, moderately isotopically-depleted source, likely to be the northwestern edge of the subtropical gyre; and a cold, isotopically enriched source, which we hypothesize to be the subpolar waters overlying the main branch of North Atlantic Deep Water flowing southwest from the Nordic Seas.  相似文献   

8.
Long-chain alkenones in lacustrine settings are potentially excellent biomarkers for the reconstruction of past terrestrial environmental conditions, and have been found in many different types of lakes around the globe. A wider range of factors influence the occurrence and distribution of alkenones in lake sediments and waters when compared to marine systems. Lake environmental conditions, such as temperature (in particular) and salinity, are among the key factors controlling alkenone distributions in lacustrine settings. Here we investigated alkenone distribution patterns in lakes of the northern Qinghai-Tibetan Plateau, China, and their possible relationship with environmental conditions, by analyzing paired samples of suspended particulate matter in surface waters and surface sediments. Salinity of investigated lake waters ranges from almost 0 to ∼100 g/L, while temperature variation among the lakes is minimal, effectively eliminating temperature effects on the alkenone distribution patterns observed here. We show that (1) alkenone concentrations vary substantially between the lakes, yet controlling mechanisms remain elusive; (2) C37/C38 ratios are substantially lower in the lakes of the Qaidam Basin than in the Lake Qinghai region, probably indicating different alkenone producers in the two regions; and (3) large variations in %C37:4 (the percentage of the C37:4 alkenone), determined from both surface waters and sediments, are negatively correlated with salinity. We suggest that the %C37:4 index could be used as a salinity indicator at least on a regional scale, with careful considerations of other potentially complicating factors. However, potential reasons for why salinity could significantly affect %C37:4 values need further investigation.  相似文献   

9.
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10.
We investigated relationships between sedimentary solvent-extractable long-chain alkenone (LCA) concentration and composition and environmental factors in a suite of endorheic lakes from inland Spain. LCAs were found in 14 of the 54 lakes examined, with concentrations comparable with those from previously published lacustrine settings. The composition of LCAs in our sites, however, contrast from the majority of those previously reported from lake environments; in our study the tri-unsaturated component is the most abundant component at most sites where LCAs are detected, and C38:3 is the most abundant LCA in the majority of sites. LCA occurrence appears to be restricted to brackish-hypersaline sites and C37 LCAs are absent above a salinity of ∼40 g L−1 suggesting a salinity control on LCA-producing organisms in these sites. Low concentrations of C37 LCA components means and temperature indices are generally not applicable. Instead we find good relationships between C38 components and (in particular mean autumn) temperature and the strongest LCA-temperature relationships are found when using a combination of all C37 and C38 compounds. We propose a new alkenone temperature index for lakes with elevated salinity and where the C38 components dominate the LCA distributions. This is expressed as (r2 = 0.80, n = 13). In this paper, we provide the first account of sedimentary LCA distributions from lakes in inland Spain, extending the range of environments within which these compounds have been found and highlighting their significance as indicators of both salinity and temperature in saline, endorheic lake environments. This has important implications for extending the potential role of LCAs as palaeoclimatic indicators in lacustrine environments.  相似文献   

11.
Our analysis of lipid molecular fossils from a Lake Titicaca (16° S, 69° W) sediment core reveals distinct changes in the ecology of the lake over an ∼25,000-yr period spanning latest Pleistocene to late Holocene time. Previous investigations have shown that over this time period Lake Titicaca was subject to large changes in lake level in response to regional climatic variability. Our results indicate that lake algal populations were greatly affected by the changing physical and chemical conditions in Lake Titicaca. Hydrocarbons are characterized by a combination of odd-numbered, mid- to long-chain (C21-C31) normal alkanes and alkenes. During periods when lake level was higher (latest Pleistocene, early Holocene, and late Holocene), the C21n-alkane, and the C25 and C27 alkenes dominate hydrocarbon distributions and indicate contribution from an algal source, potentially the freshwater alga Botryococcus braunii. The C30 4 α-methyl sterol (dinosterol) increases sharply during the mid-Holocene, suggesting a greatly increased dinoflagellate presence at that time. Long-chain alkenones (LCAs) become significant during the early Holocene and are highly abundant in mid-Holocene samples. There are relatively few published records of LCA detection in lake sediments but their occurrence is geographically widespread (Antarctica, Asia, Europe, North America). Lake Titicaca represents the first South American lake and the first low-latitude lake in which LCAs have been reported. LCA abundance and distribution may be related to the temperature-dependent response of an unidentified algal precursor. Although the LCA unsaturation indices cannot be used to determine absolute Lake Titicaca temperatures, we suspect that the published LCA U37K unsaturation calibrations can be applied to infer relative temperatures for early to mid-Holocene time when LCA concentrations are high. Using these criteria, the U37K unsaturation indices suggest relatively warmer temperatures in the mid-Holocene. In contrast to previous speculation, lipid analysis provides little evidence of a greatly increased presence of aquatic plants during the mid-Holocene. Instead, it appears that a few algal species were dominant in the lake. Based on the dramatic rise in abundances of LCAs and dinosterol during the early to mid-Holocene, we suspect that the algal producers of these compounds rose in response to a combination of physical and chemical changes in the lake. These include temperature, salinity, and alkalinity changes that occurred as lake level dropped sharply during a multi-millennial drought affecting the Central Andean Altiplano.  相似文献   

12.
The UK37′ index has proven to be a robust proxy to estimate past sea surface temperatures (SSTs) over a range of time scales, but like any other proxy, it has uncertainties. For instance, in reconstructions of the Last Glacial Maximum (LGM) in the northern North Atlantic, UK37′ indicates higher temperatures than those derived from foraminiferal proxies. Here we evaluate whether such warm glacial estimates are caused by the advection of reworked alkenones in ice‐rafted debris (IRD) to deep‐sea sediments. We have quantified both coccolith assemblages and alkenones in sediments from glaciogenic debris flows in the continental margins of the northern North Atlantic, and from a deep‐sea core from the Reykjanes Ridge. Certain debris flow deposits in the North Atlantic were generated by the presence of massive ice‐sheets in the past, and their associated ice streams. Such deposits are composed of the same materials that were present in the IRD at the time they were generated. We conclude that ice rafting from some locations was a transport pathway to the deep sea floor of reworked alkenones and pre‐Quaternary coccolith species during glacial stages, but that not all of the IRD contained alkenones, even when reworked coccoliths were present. We speculate that the ratio of reworked coccoliths to alkenone concentration might be useful to infer whether significant reworked alkenone inputs from IRD did occur at a particular site in the glacial North Atlantic. We also observe that alkenones in some of the debris flows contain a colder signal than estimated for LGM sediments in the northern North Atlantic. This is also clear in the deep‐sea core studied where the warmest intervals do not correspond to the intervals with large inputs of reworked coccoliths or IRD. We conclude that any possible bias to UK37′ estimates associated with reworked alkenones is not necessarily towards higher values, and that the high SST anomalies for the LGM are unlikely to be the result of a bias caused by IRD inputs. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

13.
Three methods are presented on how to purify acetylated sterols, acetylated triterpenols and individual alkenones for hydrogen isotope analysis from marine and lacustrine sediments using reverse-phase high performance liquid chromatography (RP-HPLC). The main advantages over previous HPLC methods are reduced operator time, increased automation and the ability to simultaneously purify multiple target compounds from a sample. These gains are achieved primarily by acetylating compounds prior to purification rather than after, and also by using a fraction collector with semi-preparatory rather than analytic configuration. The effectiveness of the method is demonstrated for (i) dinosterol and taraxerol in sediment from the brackish pond Poza del Diablo, Galápagos, (ii) for di- and tri-unsaturated C37 and C38 alkenones in cultured Emiliania huxleyi, (iii) for brassicasterol, and di-, tri- and tetra-unsaturated C37 alkenones in sediment from Manito Lake, Saskatchewan, Canada, and (iv) for brassicasterol, dinosterol and di-, tri- and tetra-unsaturated C37 alkenones in sediment from the Great Salt Lake, Utah. The purification process yields 80–90% recoveries and results in no measurable hydrogen isotope alteration.  相似文献   

14.
《Quaternary Science Reviews》2007,26(19-21):2487-2504
Quantitative paleoclimate reconstructions from Southern Hemisphere mid-latitude sites are critical for identifying spatial and temporal patterns of abrupt climate change and for testing hypotheses about possible causal mechanisms. The analysis of chironomid distributions and environmental variables from 61 lakes was undertaken to develop a robust inference model for quantitative temperature reconstruction from sites in South Island, New Zealand. Detrended correspondence analysis (DCA) and canonical correspondence analysis (CCA) indicated that mean summer air temperature (SmT), chlorophyll a, conductivity, organic content of sediment as determined by loss-on-ignition, and mean annual precipitation account for a significant portion of variance in chironomid taxa distribution. SmT was the dominant variable. The strongest performing inference model, based on weighted averaging-partial least squares (WA-PLS), was developed from 60 lakes for SmT, having a root mean square error of prediction (RMSEPjack) of 1.27 °C and a coefficient of determination of 0.80 (r2jack). These results confirm that New Zealand chironomids may be used to provide robust estimates of mean summer air temperature and are a reliable method for quantitative paleoenvironmental reconstruction.  相似文献   

15.
Hydrogen isotope ratios of individual alkenones from haptophyte algae were measured in suspended particles and surface sediment from the Chesapeake Bay (CB) estuary, eastern USA, in order to determine their relationship to water δD values and salinity. δD values of four alkenones (MeC37:2, MeC37:3, EtC38:2, EtC38:3) from particles and sediments were between −165‰ and −221‰ and increased linearly (R2 = 0.7-0.9) with water δD values from the head to the mouth of the Bay. Individual alkenones were depleted in deuterium by 156-188‰ relative to water. The MeC37 alkenones were consistently enriched by ∼12‰ relative to the EtC38 alkenones, and the di-unsaturated alkenones of both varieties were consistently enriched by ∼20‰ relative to the tri-unsaturated alkenones. All of the increase in alkenone δD values could be accounted for by the water δD increase. Consequently, no net change in alkenone-water D/H fractionation occurred as a result of the salinity increase from 10 to 29. This observation is at odds with results from culture studies with alkenone-producing marine coccolithophorids, and from two field studies, one with a dinoflagellate sterol in the CB, and one with a wide variety of lipids in saline ponds on Christmas Island, that indicate a decline in D/H fractionation with increasing salinity. Why D/H fractionation in alkenones in the CB showed no dependence on salinity, while D/H fractionation in CB dinsoterol decreased by 1‰ per unit increase in salinity remains to be determined. Two hypotheses we consider to be valid are that (i) the assemblage of alkenone-producing haptophytes changes along the Bay and each species has a different sensitivity to salinity, such that no apparent trend in αalkenone-water occurs along the salinity gradient, and (ii) greater osmoregulation capacity in coastal haptophytes may result in a diminished sensitivity of alkenone-water D/H fractionation to salinity changes.  相似文献   

16.
Lakes worldwide are commonly oversaturated with CO2, however the source of this CO2 oversaturation is not well understood. To examine the magnitude of the C flux to the atmosphere and determine if an excess of respiration (R) over gross primary production (GPP) is sufficient to account for this C flux, metabolic parameters and stable isotopes of dissolved O2 and C were measured in 23 Québec lakes. All of the lakes sampled were oversaturated with CO2 over the sampling period, on average 221 ± 25%. However, little evidence was found to conclude that this CO2 oversaturation was the result of an excess of pelagic R over GPP. In lakes Croche and à l’Ours, where CO2 flux, R and GPP were measured weekly, the annual difference between pelagic GPP and R, or net primary production (NPP), was not sufficient to account for the size of the CO2 flux to the atmosphere. In Lac Croche average annual NPP was 14.4 mg C m−2 d−1 while the average annual flux of CO2 to the atmosphere was 34 mg C m−2 d−1. In Lac à l’Ours average annual NPP was −9.1 mg C m−2 d−1 while the average annual flux of CO2 to the atmosphere was 55 mg C m−2 d−1. In all of the lakes sampled, O2 saturation averaged 104.0 ± 1.7% during the ice-free season and the isotopic composition of dissolved O2 (δ18ODO) was 22.9 ± 0.3‰, lower than atmospheric values and indicative of net autotrophy. Carbon evasion was not a function of R, nor did the isotopic signature of dissolved CO2 in the lakes present evidence of excess R over GPP. External inputs of C must therefore subsidize the lake to explain the continued CO2 oversaturation. The isotopic composition of dissolved inorganic C (δ13CDIC) indicates that the CO2 oversaturation cannot be attributed to in situ aerobic respiration. δ13CDIC reveals a source of excess C enriched in 13C, which may be accounted for by anaerobic sediment respiration or groundwater inputs followed by kinetic isotope fractionation during degassing under open system conditions.  相似文献   

17.
《Applied Geochemistry》2005,20(9):1637-1647
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18.
We present a new set of 14C ages obtained by accelerator mass spectrometry (AMS) on planktonic foraminifera from a deep-sea core collected off the Iberian Margin (MD952042). This site, at 37°N, is distant from the high-latitude zones where 14C reservoir age is large and variable. Many independent proxies — alkenones, magnetic susceptibility, ice-rafted debris, foraminifera stable isotopes, abundances of foraminifera, pollen, and dinoflagellates — show abrupt changes correlative with Dansgaard-Oeschger and Heinrich events of the last glacial period. The good stratigraphic agreement of all proxies — from the fine to the coarse-size fractions — indicates that the foraminifera 14C ages are representative of the different sediment fractions. To obtain reliable 14C ages of foraminifera beyond 20,000 14C yr B.P., we leached the shells prior to carbonate hydrolysis and subsequent analysis. For a calendar age scale, we matched the Iberian Margin U37K′ profile with that of Greenland Summit δ18O. Both are proxies for temperature, which in models varies synchronously in the two areas. The match creates no spurious jumps in sedimentation rate and requires only a limited number of tie points. Except for ages older than 40,000 14C yr B.P., Greenland's GISP2 and GRIP records yield similar calendars. The 14C and imported calendar ages of the Iberian Margin record are then compared to data — from lacustrine annual varves and from corals and speleothems dated by U-Th — previously used to extend the calibration beyond 20,000 14C yr B.P. The new record follows a smooth pattern between 23,000 and 50,000 cal yr B.P. We find good agreement with the previous data sets between 23,000 and 31,000 cal yr B.P. In the interval between 33,000 and 41,000 cal yr B.P., for which previous records disagree by up to 5000 cal yr, the Iberian Margin record closely follows the polynomial curve that was previously defined by an interpolation of the coral ages and runs between the Lake Suigetsu and the Bahamian speleothem data sets.  相似文献   

19.
The carbon stable isotopic value of dissolved inorganic carbon (δ13CDIC) was measured over several years at different depths in the water column in six carbonate-precipitating temperate lakes. δ13CDIC behavior in three of these lakes departed from the conventional model wherein epilimnetic waters are seasonally enriched relative to all hypolimnetic waters, and in general δ13CDIC values in the water column were not readily correlated to parameters such as lake stratification, algal productivity, hydraulic residence time, or water chemistry. Additionally, the processes implicated in generating the δ13CDIC values of individual lakes differ between lakes with similar δ13CDIC compositions. Each lake thus initially appears idiosyncratic, but when the effects of carbonate mineral equilibria, microbial activity, and lake residence time are viewed in terms of the magnitude of distinct DIC pools and fluxes in stratified lakes, generalizations can be made that allow lakes to be grouped by δ13CDIC behavior. We recognize three modes in the relationship between δ13CDIC values and DIC concentration ([DIC]) of individual lakes: (A) δ13CDIC values decreasing with increasing [DIC]; (B) δ13CDIC values increasing with increasing [DIC]; (C) δ13CDIC values decreasing with increasing [DIC] but increasing again at the highest [DIC]. This approach is useful both in understanding δ13CDIC dynamics in modern hardwater lakes and in reconstructing the environmental changes recorded by sedimentary δ13C components in the lacustrine paleorecord.  相似文献   

20.
High-resolution natural abundance stable carbon isotope analyses across annual growth rings in evergreen trees reveal a cyclic increase and decrease in the measured carbon isotopic composition (δ13C), but the causes of this pattern are poorly understood. We compiled new and published high-resolution δ13C data from across annual growth rings of 33 modern evergreen trees from 10 genera and 15 globally distributed sites to quantify the parameters that affect the observed δ13C pattern. Across a broad range of latitude, temperature, and precipitation regimes, we found that the average, measured seasonal change in δ13C (Δδ13Cmeas, ‰) within tree rings of evergreen species reflects changes in the carbon isotopic composition of atmospheric carbon dioxide (Δδ13CCO2) and changes in seasonal precipitation (ΔP) according to the following equation: Δδ13Cmeas = Δδ13CCO2 - 0.82(ΔP) + 0.73; R2 = 0.96. Seasonal changes in temperature, pCO2, and light levels were not found to significantly affect Δδ13Cmeas. We propose that this relationship can be used to quantify seasonal patterns in paleoprecipitation from intra-ring profiles of δ13C measured from non-permineralized, fossil wood.  相似文献   

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