We investigated the provenance of organic matter in the inner fjord area of northern Patagonia, Chile (∼44–47°S), by studying the elemental (organic carbon, total nitrogen), isotopic (δ
13C, δ
15N), ...and biomarker (
n-alkanoic acids from vascular plant waxes) composition of surface sediments as well as local marine and terrestrial organic matter. Average end-member values of N/C, δ
13C, and δ
15N from organic matter were 0.127±0.010, −19.8±0.3‰, and 9.9±0.5‰ for autochthonous (marine) sources and 0.040±0.018, −29.3±2.1‰, and 0.2±3.0‰ for allochthonous (terrestrial) sources. Using a mixing equation based on these two end-members, we calculated the relative contribution of marine and terrestrial organic carbon from the open ocean to the heads of fjords close to river outlets. The input of marine-derived organic carbon varied widely and accounted for 13–96% (average 61%) of the organic carbon pool of surface sediments. Integrated regional calculations for the inner fjord system of northern Patagonia covered in this study, which encompasses an area of ∼4280
km
2, suggest that carbon accumulation may account for between 2.3 and 7.8×10
4
ton
C
yr
−1. This represents a storage capacity of marine-derived carbon between 1.8 and 6.2×10
4
ton
yr
−1, which corresponds to an assimilation rate of CO
2 by marine photosynthesis between 0.06 and 0.23×10
6
ton
yr
−1. This rate suggests that the entire fjord system of Patagonia, which covers an area of ∼240,000
km
2, may represent a potentially important region for the global burial of marine organic matter and the sequestration of atmospheric CO
2.
Climate and the Collapse of Maya Civilization Haug, Gerald H.; Günther, Detlef; Peterson, Larry C. ...
Science (American Association for the Advancement of Science),
03/2003, Letnik:
299, Številka:
5613
Journal Article
Recenzirano
In the anoxic Cariaco Basin of the southern Caribbean, the bulk titanium content of undisturbed sediment reflects variations in riverine input and the hydrological cycle over northern tropical South ...America. A seasonally resolved record of titanium shows that the collapse of Maya civilization in the Terminal Classic Period occurred during an extended regional dry period, punctuated by more intense multiyear droughts centered at approximately 810, 860, and 910 A.D. These new data suggest that a century-scale decline in rainfall put a general strain on resources in the region, which was then exacerbated by abrupt drought events, contributing to the social stresses that led to the Maya demise.
•Controls on GDGT distributions in Arctic lakes are studied.•GDGTs are ubiquitous in Arctic lakes.•Strong correlation between branched GDGTs and warm season temperature.•Correlations are consistent ...with bGDGT–temperature relationships in the tropics.•Data suggest bGDGTs are produced mostly in summer.
Understanding the causes and potential ecological and environmental impacts of recent and future temperature changes in the Arctic requires a better understanding of long term climate variability. Amongst the most promising new geochemical tools for sedimentary paleotemperature reconstructions are those based on the glycerol diakyl glycerol tetraethers (GDGTs). Although a number of studies have recently developed empirical calibrations for the temperature sensitivity of isoprenoid GDGTs (i.e., the TEX86 index) and the branched GDGTs (i.e., the MBT/CBT index) in mid and low latitude lakes, there is very little data from lakes in the Arctic. Here, we examine the temperature and environmental controls on GDGT abundances in a transect of small lakes (n=59) distributed across Baffin Island in the eastern Canadian Arctic. Isoprenoid and branched GDGTs are ubiquitous in these lakes. The temperature control on the isoprenoid GDGT distributions is weak, although there is a significant relationship between the branched GDGT distributions and warm season temperature. Furthermore, published relationships developed between branched GDGTs and mean annual air temperatures in tropical lakes yield reconstructed temperatures from Arctic lakes that are consistent with Arctic summer air temperature. This suggests that empirical calibrations from branched GDGTs in lakes reflect the seasonality of branched GDGT production. For example, at low latitudes there is little seasonality in temperature, and branched GDGT production records air temperatures throughout the year. In the Arctic however, branched GDGTs are likely produced in summer, when the temperatures are warmest, sunlight hours are greatest and ice cover is diminished. Due to the extreme seasonality and short window of Arctic productivity, the use of branched GDGTs for summer paleotemperature reconstructions likely remains robust back through time. However, interpretations of paleotemperatures from branched GDGTs in temperate lakes may require careful consideration about potential changes in the seasonal timing of branched GDGT production.
Stable isotope analysis of leaf waxes in a sediment core from Laguna La Gaiba, a shallow lake located at the Bolivian margin of the Pantanal wetlands, provides new perspective on vegetation and ...climate change in the lowland interior tropics of South America over the past 40,000 years. The carbon isotopic compositions (δ13C) of long-chain n-alkanes reveal large shifts between C3- and C4-dominated vegetation communities since the last glacial period, consistent with landscape reconstructions generated with pollen data from the same sediment core. Leaf wax δ13C values during the last glacial period reflect an open landscape composed of C4 grasses and C3 herbs from 41–20 ka. A peak in C4 abundance during the Last Glacial Maximum (LGM, ∼21 ka) suggests drier or more seasonal conditions relative to the earlier glacial period, while the development of a C3-dominated forest community after 20 ka points to increased humidity during the last deglaciation. Within the Holocene, large changes in the abundance of C4 vegetation indicate a transition from drier or more seasonal conditions during the early/mid-Holocene to wetter conditions in the late Holocene coincident with increasing austral summer insolation. Strong negative correlations between leaf wax δ13C and δD values over the entire record indicate that the majority of variability in leaf wax δD at this site can be explained by variability in the magnitude of biosynthetic fractionation by different vegetation types rather than changes in meteoric water δD signatures. However, positive δD deviations from the observed δ13C–δD trends are consistent with more enriched source water and drier or more seasonal conditions during the early/mid-Holocene and LGM. Overall, our record adds to evidence of varying influence of glacial boundary conditions and orbital forcing on South American Summer Monsoon precipitation in different regions of the South American tropics. Moreover, the relationships between leaf wax stable isotopes and pollen data observed at this site underscore the complementary nature of pollen and leaf wax δ13C data for reconstructing past vegetation changes and the potentially large effects of such changes on leaf wax δD signatures.
•Climate history of Pantanal wetlands over past 40 kyr from leaf wax δ13C and δD.•Leaf wax δ13C reveals large C3/C4 vegetation shifts complementing pollen record.•Drier LGM conditions point to regional variability of glacial monsoon precipitation.•Holocene hydrologic trend consistent with precessional forcing of monsoon.•Leaf wax δD record mainly reflects vegetation change.
The Patagonian fjords have a clear potential to provide high-resolution sedimentary and geochemical records of past climate and environmental change in the Southern Andes. To improve our ability to ...interpret these proxy records, we investigated the processes that control fjord sediment inorganic geochemistry through a geochemical, mineralogical and sedimentological analysis of surface sediment samples from the fjords of Northern Chilean Patagonia. A simple Terrestrial Index based on measurements of salinity and Fraction of Terrestrial Carbon was used to estimate the terrestrial input/river discharge at each site. Our results demonstrate that, under the cold climate conditions of Patagonia, chemical weathering is weak and the inorganic geochemical composition of the fjord sediments is primarily controlled by hydrodynamic mineralogical sorting, i.e., the intensity of river discharge. Our results suggest that the distribution of Fe, Ti and Zr in surface sediments is controlled by their association with heavy and/or coarse minerals, whereas Al is independent of hydrodynamic processes. The elemental ratios Fe/Al, Ti/Al and Zr/Al are therefore well suited for estimating changes in the energy of terrestrial sediment supply into the fjords through time. Zr/Al is particularly sensitive in proximal environments, while Fe/Al is most useful in the outer fjords and on the continental margin. In the most proximal environments, however, Fe/Al is inversely related to hydrodynamic conditions. Caution should therefore be exercised when interpreting Fe/Al ratios in terms of past river discharge. The application of these proxies to long sediment cores from Quitralco fjord and Golfo Elefantes validates our interpretations. Our results also emphasize the need to measure Al-based elemental ratios at high precision, which can be achieved using simultaneous acquisition ICP-AES technology. This study therefore constitutes a strong basis for the interpretation of sedimentary records from the Chilean Fjords.
A high-resolution western tropical Atlantic sea surface temperature (SST) record from the Cariaco Basin on the northern Venezuelan shelf, based on Mg/Ca values in surface-dwelling planktonic ...foraminifera, reveals that changes in SST over the last glacial termination are synchronous, within ± 30 to ±90 years, with the Greenland Ice Sheet Project 2 air temperature proxy record and atmospheric methane record. The most prominent deglacial event in the Cariaco record occurred during the Younger Dryas time interval, when SSTs dropped by 3° to 4°C. A rapid southward shift in the atmospheric intertropical convergence zone could account for the synchroneity of tropical temperature, atmospheric methane, and high-latitude changes during the Younger Dryas.
Sedimentary time series of color reflectance and major element chemistry from the anoxic Cariaco Basin off the coast of northern Venezuela record large and abrupt shifts in the hydrologic cycle of ...the tropical Atlantic during the past 90,000 years. Marine productivity maxima and increased precipitation and riverine discharge from northern South America are closely linked to interstadial (warm) climate events of marine isotope stage 3, as recorded in Greenland ice cores. Increased precipitation at this latitude during interstadials suggests the potential for greater moisture export from the Atlantic to Pacific, which could have affected the salinity balance of the Atlantic and increased thermohaline heat transport to high northern latitudes. This supports the notion that tropical feedbacks played an important role in modulating global climate during the last glacial period.
The Sr/Ca ratio of coral aragonite is used to reconstruct past sea surface temperature (SST). Twenty‐one laboratories took part in an interlaboratory study of coral Sr/Ca measurements. Results show ...interlaboratory bias can be significant, and in the extreme case could result in a range in SST estimates of 7°C. However, most of the data fall within a narrower range and the Porites coral reference material JCp‐1 is now characterized well enough to have a certified Sr/Ca value of 8.838 mmol/mol with an expanded uncertainty of 0.089 mmol/mol following International Association of Geoanalysts (IAG) guidelines. This uncertainty, at the 95% confidence level, equates to 1.5°C for SST estimates using Porites, so is approaching fitness for purpose. The comparable median within laboratory error is <0.5°C. This difference in uncertainties illustrates the interlaboratory bias component that should be reduced through the use of reference materials like the JCp‐1. There are many potential sources contributing to biases in comparative methods but traces of Sr in Ca standards and uncertainties in reference solution composition can account for half of the combined uncertainty. Consensus values that fulfil the requirements to be certified values were also obtained for Mg/Ca in JCp‐1 and for Sr/Ca and Mg/Ca ratios in the JCt‐1 giant clam reference material. Reference values with variable fitness for purpose have also been obtained for Li/Ca, B/Ca, Ba/Ca, and U/Ca in both reference materials. In future, studies reporting coral element/Ca data should also report the average value obtained for a reference material such as the JCp‐1.
Key Points
Twenty‐one labs have characterised the coral JCp‐1
The 95% confidence level equates to 1.5{degree sign}C for SST estimates
Uncertainty can be reduced through the use of reference materials like JCp‐1
The hydrogen isotope composition of plant waxes preserved in lacustrine sediments is a potentially valuable tool for reconstructing paleoenvironmental changes in the Arctic. However, in contrast to ...the mid- and low-latitudes, significantly less effort has been directed towards understanding the factors controlling D/H fractionation in high latitude plant waxes and the impact of these processes on the interpretation of sedimentary leaf wax δD records. To better understand these processes, we examined the D/H ratios of long chain fatty acids in lake surface sediments spanning a temperature and precipitation gradient on Baffin Island in the eastern Canadian Arctic. D/H ratios of plant waxes increase with increasing temperature and aridity, with values ranging from −240‰ to −160‰ over the study area. Apparent fractionation factors between n-alkanoic acids in Arctic lake sediments and precipitation(εFA-ppt) are less negative than those of mid-latitude lakes and modern plants by 25‰ to 65‰, consistent with n-alkane data from modern Arctic plants (Yang et al., 2011). Furthermore, εFA-ppt values from Arctic lakes become systematically more positive with increasing evaporation, in contrast to mid-latitude sites, which show little to no change in fractionation with aridity. These data are consistent with enhanced water loss and isotope fractionation at higher latitude in the Arctic summer, when continuous sunlight supports increased daily photosynthesis. The dominant control on δDFA variations on Baffin Island is temperature. However, changing εFA-ppt result in steeper δDFA–temperature relationships than observed for modern precipitation. The application of this δDFA-based paleotemperature calibration to existing δDFA records from Baffin Island produces much more realistic changes in late Holocene temperature and highlights the importance of these effects in influencing the interpretation of Arctic δDFA records. A better understanding of the controls on hydrogen isotope fractionation in high latitude leaf waxes will be essential to the proper interpretation of isotope records from sedimentary plant waxes in the Arctic.
Identifying leads and lags between high- and low-latitude abrupt climate shifts is needed to understand where and how such events were triggered. Vascular plant biomarkers preserved in Cariaco basin ...sediments reveal rapid vegetation changes in northern South America during the last deglaciation, 15,000 to 10,000 years ago. Comparing the biomarker records to climate proxies from the same sediment core provides a precise measure of the relative timing of changes in different regions. Abrupt deglacial climate shifts in tropical and high-latitude North Atlantic regions were synchronous, whereas changes in tropical vegetation consistently lagged climate shifts by several decades.
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