Increase In Sediment Accumulation Rates Research Articles

East Asian summer monsoon variations across the Miocene−Pliocene boundary recorded by sediments from the Guide Basin, northeastern Tibetan Plateau

Records of atmospheric carbon dioxide concentration, sea-surface temperature, and global vegetation show that Earth’s climate and environment changed significantly during the late Miocene−early Pliocene. Understanding the environmental response to insolation forcing during this transitional period may provide insights into future environmental variations resulting from the perturbation of the global carbon cycle caused by fossil fuel combustion. However, terrestrial paleoclimate records capable of resolving orbital time-scale environmental variations are mostly from Europe, especially from the region around the Mediterranean Sea. Here, we present high-resolution records of grain size, black carbon, and geochemistry from a sedimentary sequence from the northeastern margin of the Tibetan Plateau, where precipitation is mainly via the East Asian summer monsoon. We observed increases in sediment accumulation rate and black carbon mass accumulation rate at ca. 5.3 Ma, which we interpret as the result of intensified seasonal precipitation associated with the strengthening of the East Asian summer monsoon; concurrently, precessional and obliquity cycles became more prominent during the early Pliocene. Our results suggest that, in response to current and future high atmospheric carbon dioxide concentrations, changes in the East Asian summer monsoon are likely to result in increased precipitation and seasonality within its region of influence.

Sediment Infill of Tropical Floodplain Lakes: Rates, Controls, and Implications for Ecosystem Services

Shallow lakes in tropical floodplains provide significant ecosystem services that can be altered by natural and anthropogenic forces. Despite their importance, little is known about the infill patterns and timescales and the magnitude of these changes in tropical floodplain lakes. Here, we present a global meta-analysis of sediment core-derived accumulation rate data for shallow floodplain lakes in tropical lowlands to quantify the timescales of basin infill. Environmental variables (e.g., sediment accumulation rates, bathymetry, surface area) were compiled from the literature or derived from remote sensing imagery, resulting in a database (n = 76 lakes) that includes various lake morphologies. Our results show an exponential increase in sediment accumulation rates in many of these lakes over the past 50 years, which we interpret as a response to growing human populations and deforestation, particularly in topographically steep watersheds with pronounced seasonal rainfall. Over centennial periods, tropical floodplain lakes accumulate sediment faster than many other extratropical lakes. The dataset suggests that complete infill of some tropical floodplain lakes will occur in as little as a few centuries. Our findings also reveal the critical environmental and human factors that influence sediment accumulation patterns and affect ecosystem services in shallow tropical floodplain lakes. These findings have important implications for water and sediment management in low latitude watersheds, many of which are located in densely populated and/or developing nations.

Rain Forest Fragmentation and Environmental Dynamics on Nosy Be Island (NW Madagascar) at 1300 cal BP Is Attributable to Intensified Human Impact

Madagascar houses one of the Earth’s biologically richest, but also one of most endangered, terrestrial ecoregions. Although it is obvious that humans substantially altered the natural ecosystems during the past decades, the timing of arrival of early inhabitants on Madagascar as well as their environmental impact is still intensively debated. This research aims to study the beginning of early human impact on Malagasy natural ecosystems, specifically on Nosy Be island (NW Madagascar) by targeting the sedimentary archive of Lake Amparihibe, an ancient volcanic crater. Based on pollen, fungal spore, other non-pollen palynomorph, charcoal particle and diatom analyses combined with high-resolution sediment-physical and (in)organic geochemical data, paleoenvironmental dynamics during the past three millennia were reconstructed. Results indicate a major environmental change at ca. 1300 cal BP characterized by an abrupt development of grass (C4) dominated and fire disturbed landscape showing the alteration of natural rain forest. Further, increased soil erodibility is suggested by distinct increase in sediment accumulation rates, a strong pulse of nutrient input, higher water turbidity and contemporaneous increase in spores of mycorrhizal fungi. These parameters are interpreted to show a strong early anthropogenic transformation of the landscape from rain forest to open grassland. After ca. 1000 cal BP, fires remain frequent and vegetation is dominated by forest/grassland mosaic. While natural vegetation should be dominated by rain forest on Nosy Be, these last results indicate that human continuously impacted the landscapes surrounding the lake. At a local scale, our data support the “subsistence shift hypothesis” which proposed that population expansion with development of herding/farming altered the natural ecosystems. However, a precise regional synthesis is challenging, since high-resolution multi-proxy records from continuous sedimentary archives as well as records located further north and in the hinterland are still scarce in Madagascar. The lack of such regional synthesis also prevents precise comparison between different regions in Madagascar to detect potential (dis)similarities in climate dynamics, ecosystem responses and anthropogenic influences at the island’s scale during the (late) Holocene.

Effects of bottom trawling on trace metal contamination of sediments along the submarine canyons of the Gulf of Palermo (southwestern Mediterranean)

Submarine canyons are preferential pathways for transport of particulate matter and contaminants from the shelf to the deep sea. The Gulf of Palermo continental margin has a very narrow shelf (about 2–3 km wide on average) and is incised by several submarine canyons that favour shelf-slope sediment transfer. A sediment core collected on the outer shelf and six sediment cores taken at different depths along the Oreto, Eleuterio and Anerella submarine canyons were analysed to study the transfer and historical record of trace metal contamination in the Gulf of Palermo continental margin. Trace metals, major elements, organic carbon and sediment grain size were analysed in these cores, which were dated with 210Pb to assess their historical compositional evolution since the late 19th century. Hg, Pb, Cu, Zn and Cd content increased until the 1970s and 1980s, associated with the increase in urbanization and industrial activities in the Palermo area, and Hg was the contaminant that reached the highest enrichments. However, the increasing trend of these metals contamination was reversed in the 1970s and 1980s, coinciding with drastic changes in the terrigenous content and grain size of sediments in the canyon axes. These changes occurred when bottom trawling fleets expanded to deeper fishing grounds equipped with powerful trawlers around the Gulf of Palermo canyon heads and flanks and along the Oreto canyon axis. Bottom trawlers have resuspended large amounts of sediment, which have been transferred into the canyons since the 1970s and 1980s and have thus increased sediment accumulation rates. This resuspended sediment has been mixing with the sediment transferred and accumulated along the canyons, diluting and reducing its trace metal contamination levels since the expansion of the bottom trawling fleets.

Erosion and sedimentation in SE Tibet and Myanmar during the evolution of the Burmese continental margin from the Late Cretaceous to Early Neogene

The evolution of large fluvial drainages in SE Tibet and adjacent areas reflects the degree of coupling between tectonics, erosion, and climate. Reconstruction of their spatio-temporal development provides significant constraints for our understanding of the palaeogeographic evolution of numerous blocks in this domain. The Burmese continental margin drained by the Irrawaddy River and its tributaries includes blocks of the Burma Terrane and the Mount Victoria Land (Eastern Belt of the Indo-Burman Ranges). To investigate how was this margin developed from the amalgamation of the two blocks since the mid-Cretaceous, we present seismic profiles, sedimentary budget, sandstone petrography, bulk-rock geochemistry, and detrital apatite U-Pb ages from the margin. Our results show that provenance of the Burmese Forearc and Backarc depressions shifted from the proximal Western Myanmar Arc and equivalents within northern Burma Terrane to the northern Mogok Metamorphic Belt and Dianxi-Burman batholiths within SE Tibet during the late Paleogene. The provenance transition is synchronous with tectonic evolution of the Burma Terrane from an(a) oceanic/continental island arc setting to an active continental margin. We propose an evolved Western Pacific-type margin for the Burma Terrane, which is dominated by typical accretionary wedge, calc-alkaline island arc, and extensional forearc/backarc basins. An increase in sediment accumulation rates (SARs) in the Burmese Forearc indicates that the input from basement rocks within northern Burma Terrane (Tagaung–Myitkyina Belt) became more prominent and the Eastern Belt of the northern Indo-Burman Ranges were totally exposed in the Middle-Late Eocene. This result corroborates the Burma Terrane assembled with SE Asia (Sibumasu Terrane) before the Middle Eocene. The first presence of SE Tibet-like detritus and the diversification of SARs in the Burmese Forearc and Backarc in the Oligocene are compatible with dextral motion on the Sagaing Fault, which may have triggered the birth of the palaeo-Irrawaddy river and fluvial drainage reorganization in its upper reaches. Higher sedimentary fluxes recorded in the Burmese Forearc and Backarc depressions and the Andaman Sea during the Neogene are compatible with enhanced rock erosion within SE Tibet and thus, can be explained by rapid tectonic uplift and an intensified Asia monsoon in this region.

Historic and contemporary anthropogenic effects on granulometry and species composition detected from sediment cores and death assemblages, Nelson Bays, Aotearoa-New Zealand

To effectively manage anthropogenic stressors causing widespread and pervasive habitat change, resource managers and policy makers require advice on priority stressors to optimally target conservation and restoration outcomes. This is difficult in soft sediment ecosystems affected by multiple stressors operating across centuries with possible legacy interactions. Using tools from the emerging discipline of conservation paleobiology, we attempted to disentangle the effects of two stressors: i) changes to sediments (sedimentation rate, composition) and ii) fishing disturbance across pre-human to contemporary timescales by analysing death assemblages (DA) at a rare location protected from power fishing methods for ca. 30 yr in Nelson Bays, Aotearoa-New Zealand. Replicate sediment cores and surface grab samples were collected at paired stations, spread across three depth profiles (25, 30, 35 m), split inside and outside the fishing exclusion zone. Sediment core geochronologies were established using radioisotope dating and analysis of terrestrial plant palynomorphs (spores, pollen, starch) associated with Māori and European introduction to Aotearoa-New Zealand. Results unearthed a complex of brachiopods, molluscs, crustacea, bryozoan, and Māori-introduced pollen and starch remains – the latter a marine first. Following human arrival in the region (ca.1500AD) statistical models revealed separation of DAs between time periods were correlated with a 10–15% increase in sediment silt content following a ca.11-fold increase in Sediment Accumulation Rates (SAR). In contrast, separation of recent surface DAs (ca. 60yr) are now more strongly correlated with sediment mixing, detected by discordance in radioisotope profiles, consistent with exposure to homogenising effects of fishing disturbance outside the exclusion zone. We conclude that historic legacy effects, ongoing sedimentation and fishing disturbance have synergistically modified the sediment characteristics from “natural” baseline conditions. Those changes have likely contributed to the collapse and lack of recovery of Nelson Bays shellfish fisheries.

Spatial correlation bias in late-Cenozoic erosion histories derived from thermochronology.

The potential link between erosion rates at the Earth's surface and changes in global climate has intrigued geoscientists for decades1,2 because such a coupling has implications for the influence of silicate weathering3,4 and organic-carbon burial5 on climate and for the role of Quaternary glaciations in landscape evolution1,6. A global increase in late-Cenozoic erosion rates in response to a cooling, more variable climate has been proposed on the basis of worldwide sedimentation rates7. Other studies have indicated, however, that global erosion rates may have remained steady, suggesting that the reported increases in sediment-accumulation rates are due to preservation biases, depositional hiatuses and varying measurement intervals8-10. More recently, a global compilation of thermochronology data has been used to infer a nearly twofold increase in the erosion rate in mountainous landscapes over late-Cenozoic times6. It has been contended that this result is free of the biases that affect sedimentary records11, although others have argued that it contains biases related to how thermochronological data are averaged12 and to erosion hiatuses in glaciated landscapes13. Here we investigate the 30 locations with reported accelerated erosion during the late Cenozoic6. Our analysis shows that in 23 of these locations, the reported increases are a result of a spatial correlation bias-that is, combining data with disparate exhumation histories, thereby converting spatial erosion-rate variations into temporal increases. In four locations, the increases can be explained by changes in tectonic boundary conditions. In three cases, climatically induced accelerations are recorded, driven by localized glacial valley incision. Our findings suggest that thermochronology data currently have insufficient resolution to assess whether late-Cenozoic climate change affected erosion rates on a global scale. We suggest that a synthesis of local findings that include location-specific information may help to further investigate drivers of global erosion rates.

Peruvian Altiplano Stratigraphy Highlights Along‐Strike Variability in Foreland Basin Evolution of the Cenozoic Central Andes

AbstractRetroarc foreland basins in the Andean plateau contain critical information on geodynamic processes driving plateau development by providing a record of exhumation and sediment sourcing, as well as the timing, location, and magnitude of basin subsidence. However, this record is incomplete along orogenic strike and particularly limited in southern Peru. We measured ~6,200 m of nonmarine clastic strata in the northern Peruvian Altiplano, documented through lithofacies characterization and paleocurrent analysis, conglomerate clast counts, sandstone petrography, and detrital zircon U‐Pb geochronology; for the latter we employ quantitative detrital zircon interpretation methods including multidimensional scaling, mixture modeling, and quantification of zircon roundness. Results show dominant sediment sourcing from the Western Cordillera and/or western Altiplano. Sediment accumulation rates define an upward‐convex Paleogene subsidence profile consistent with deposition in a northeastward‐migrating flexural foreland basin system, with lithospheric loading from an increasingly proximal Western Cordilleran hinterland. Basin deposition following a 23–9 Ma angular unconformity shows a marked increase in sediment accumulation rates >800 m/Myr, interpreted as a departure from flexural subsidence. Results highlight along‐strike variability in Andean foreland basin evolution, as foredeep deposits are thicker, and the onset of rapid sediment accumulation occurs earlier in southern Peru compared to Bolivia and Argentina. Results tentatively support models of orogenic cyclicity and reveal that episodes of high‐flux magmatism in southern Peru are slightly out of phase with those documented in northwest Argentina, which may be controlled by preexisting Paleozoic‐Mesozoic structural and stratigraphic fabrics and the rate of underthrusting of melt‐fertile continental lower crust and mantle lithosphere.

Holocene alluvial sequence in the Val de Zaragoza (Los Monegros) in the palaeoenvironmental context of the Ebro Basin (NE Spain)

Flat-bottomed valleys formed by Holocene fills are the most characteristic landforms in the center of the Ebro basin. This paper analyzes, within a regional framework, a small fluvial basin located in the north of the Alcubierre Range. Three Holocene morphosedimentary units (H1-H3) and a sub-functional level (H4) are identified. These units are representative of the regional geomorphological evolutionary process in the Ebro basin. New chronological insights on climate and landscape management are provided by a study of the Bastarás weir, a construction made for water flow management during the Little Ice Age (LIA). The integration of Val de Zaragoza radiocarbon datings into the regional framework enables the authors to identify the main formation stages, as well as the influence of climatic and human factors on its evolution. The intensification of human pressure in the territory is reflected in a progressive increase in sediment accumulation rates in the valleys, peaking during Roman times and the Little Ice Age.

Amplified environmental change: Evidence from land‐use and climate change in medieval Minorca

AbstractThe debate on human environmental impact has often been locked into cause–effect reasoning, aiming at factoring human impact on top of climatic variability. Here we use evidence from Minorca and the Mediterranean region to show the potential for amplified environmental change emerging from complex feedbacks between climatic and historical events. Alluvial sediments collected in a gully reveal a 14‐ to 27‐fold increase in sediment accumulation rates, leading to the rapid aggradation of the valley floor from approximately ad 1300 onwards. These environmental changes coincided with the Feudal conquest of Minorca (ad 1287) and with the climatic shift from the Medieval Climate Anomaly (MCA, c. ad 900–1300) to the Little Ice Age (LIA, c. ad 1300–1850). This evidence of unprecedented sediment mobilisation, in context with climatic and historical events marking the Mediterranean region, highlights the implications for environmental vulnerability emerging from positive feedbacks between climate and land‐use. Understanding such interactions in historical contexts is paramount to increase our capacity for anticipatory learning in the face of rapid climatic, economical, and ecological transformations today.

High sedimentation rates and thrust fault modulation: Insights from ocean drilling offshore the St. Elias Mountains, southern Alaska

The southern Alaskan margin offshore the St. Elias Mountains has experienced the highest recorded offshore sediment accumulation rates globally. Combined with high uplift rates, active convergence and extensive temperate glaciation, the margin provides a superb setting for evaluating competing influences of tectonic and surface processes on orogen development. We correlate results from Integrated Ocean Drilling Program (IODP) Expedition 341 Sites U1420 and U1421 with regional seismic data to determine the spatial and temporal evolution of the Pamplona Zone fold-thrust belt that forms the offshore St. Elias deformation front on the continental shelf. Our mapping shows that the pattern of active faulting changed from distributed across the shelf to localized away from the primary glacial depocenter over ∼300–780 kyrs, following an order-of-magnitude increase in sediment accumulation rates. Simple Coulomb stress calculations show that the suppression of faulting is partially controlled by the change in sediment accumulation rates which created a differential pore pressure regime between the underlying, faulted strata and the overlying, undeformed sediments.

A multiproxy study distinguishes environmental change from diagenetic alteration in the recent sedimentary record of the inner Cadiz Bay (SW Spain)

In this study, we reconstruct the recent environmental evolution of the inner Cadiz Bay using sedimentary records reaching back as far as AD 1700. We report lithological descriptions of the sediments and extensive mineralogical and geochemical analyses. An extraction technique that identifies different Fe phases provides an assessment of diagenetic alteration, which allows an estimation of the original organic matter inputs to the inner Cadiz Bay. Downcore variations in Corg/N ratios, δ13Corg and δ15N are related to changes in organic matter sources and the trophic state of the water column. The downcore records of selected trace metals (e.g. Pb, Zn and Cu) are interpreted to reflect changes in heavy metal pollution in the bay, while records of other elements (e.g. Mn and P) are likely overprinted by diagenetic alteration. Major environmental shifts took place during the 20th century, when the population around Cadiz Bay increased exponentially. Increases in sediment accumulation rates, organic matter inputs and heavy metal contents, in parallel with increases in δ13Corg and δ15N over this period, are interpreted as direct effects of the increasing anthropogenic influence in the area. The results of this study suggest that multiproxy approaches and detailed consideration of diagenetic overprinting are required to reconstruct past environmental conditions from coastal sediments.

Sedimentation in dryland river waterholes: a threat to aquatic refugia?

In dryland river systems subject to prolonged low and no flow periods, waterholes, or sections of river channel that are deep relative to the rest of the channel and that retain water for longer periods of no flow, provide refugia for aquatic biota and hence are critical to the resilience of aquatic ecosystems. This study examined physical, chemical and bio-stratigraphy in refugial waterholes situated along four distributaries of the Lower Balonne River system in semi-arid Australia. In doing so we reconstructed environmental histories for the waterholes, calculated how sedimentation rates have changed in response to land use change over the past two centuries, and assessed whether they are threatened by increased sedimentation through potential effects on waterhole depth and hence persistence times and habitat quality. Our study found that sedimentation rates have increased substantially since European settlement, most likely in response to removal of groundcover by grazers. The increased sediment accumulation rates are estimated to have reduced persistence times during low and no flow periods of the waterholes by 2–4 months. Despite evidence from other similar systems in Australia that increased influx of sediment coincided with loss of submerged macrophytes, stratigraphic records of preserved pollen and diatoms did not provide consistent evidence of biotic or habitat quality changes within the waterholes associated with European settlement.

Multiple sediment cores from reservoirs are needed to reconstruct recent watershed changes from stable isotopes (δ13C and δ15N) and C/N ratios: case studies from the mid-western United States

Watershed land-use changes, including urbanization, have caused accelerated soil erosion, which in turn has increased sediment accumulation rates in most aging reservoirs in the mid-western United States (USA), thereby posing a threat to their longevity and productivity. We analyzed three sediment cores from each of two reservoirs in the mid-western USA, the Webber Dam impoundment in Michigan and Goshen Pond in Indiana. Core chronologies were established using 137Cs and 210Pb. Concentrations of organic carbon and nitrogen and their stable isotope compositions (δ13C and δ15N) were measured to investigate past land-use changes. We used three excess 210Pb-based (210Pbxs) sedimentation rate models (constant 210Pb flux and sedimentation, constant rate of supply of 210Pb and constant initial 210Pb concentration) to develop core chronologies and compared them to the chronology obtained from the 137Cs peak. Validation of the excess 210Pb-based chronology with137Cs-based chronology requires more than one 210Pbxs-based-model. That is, even in one watershed, output from more than one 210Pbxs-based model may have to be compared with an independent time marker. We compared C/N ratios, δ13C and δ15N among three time horizons in each of the three cores from the reservoirs (corresponding to dates of 1950, 1970 and 2010). Within a reservoir, there can be large differences between cores for a given time horizon, indicating there could be spatial differences in sources of C and N and the processes that influence carbon and nitrogen isotopes. We conclude that multiple sediment cores from reservoirs should be analyzed if carbon and nitrogen isotopes in sediments are to be used for paleoenvironmental inference.

Seagrass sediments reveal the long-term deterioration of an estuarine ecosystem.

The study of a Posidonia australis sediment archive has provided a record of ecosystem dynamics and processes over the last 600years in Oyster Harbour (SW Australia). Ecosystem shifts are a widespread phenomenon in coastal areas, and this study identifies baseline conditions and the time-course of ecological change (cycles, trends, resilience and thresholds of ecosystem change) under environmental stress in seagrass-dominated ecosystem. The shifts in the concentrations of chemical elements, carbonates, sediments <0.125mm and stable carbon isotope signatures (δ(13) C) of the organic matter were detected between 1850s and 1920s, whereas the shift detected in P concentration occurred several decades later (1960s). The first degradation phase (1850s-1950s) follows the onset of European settlement in Australia and was characterized by a strong increase in sediment accumulation rates and fine-grained particles, driven primarily by enhanced run-off due to land clearance and agriculture in the catchment. About 80% of total seagrass area at Oyster Harbour was lost during the second phase of environmental degradation (1960s until present). The sharp increase in P concentration and the increasing contribution of algae and terrestrial inputs into the sedimentary organic matter pool around 1960s provides compelling evidence of the documented eutrophication of the estuary and the subsequent loss of seagrass meadows. The results presented demonstrate the power of seagrass sedimentary archives to reconstruct the trajectories of anthropogenic pressures on estuarine ecosystem and the associated regime shifts, which can be used to improve the capacity of scientists and environmental managers to understand, predict and better manage ecological change in these ecosystems.

Reconstruction of historic soil erosion rates in a small lake catchment of the Melrakkaslétta peninsula of northeast Iceland using tephrochronology and ground-based LiDAR data

Erosion rates within the small catchment of an endorheic basin located in the upper Fremridalur (NE-Iceland) were quantified using a combination of borehole data, tephrochronology, terrestrial LiDAR and GIS techniques. Despite the great distance of the study area to the volcanic centers active during the Holocene, it was possible to identify five tephra layers to be used for a dating of stratigraphical units. A continuous increase in sediment accumulation rates during the last c. 6,100 years is observed. The total volume of the sediment within the basin and the sediment yield of the catchment were determined using a digital elevation model derived from terrestrial laser scanning and a modelled layer of underlying bedrock. The sediment yield within the 426 ha catchment has risen from c. 0.172 t ha–1 yr–1 during the late subboreal period (3,830–2,855 14C BP) to c. 0.493 t ha–1 yr–1 in the subatlantic period. In order to assess the present state of the study area concerning the vegetation cover and susceptibility to further erosion, the vegetation cover was mapped and a digital relief analysis was conducted. This revealed a significant degradation of the vegetation cover with increasing elevation.

Annual to millennial record of sediment delivery to US Virgin Island coastal environments

Over 100 sediment cores were collected from US Virgin Island coastal marine and salt pond environments to document the record of sediment delivery to the coastal system on annual to millennial time scales, and the extent to which human activities have influenced sedimentation patterns. Cores were analyzed for sedimentology, geochronology (210Pb, 137Cs, 7Be and 14C), and high-resolution elemental composition (scanning x-ray fluorescence (XRF) and laser ablation inductively coupled mass spectrometry (LA–ICP–MS)). The centennial–millennial record reveals the natural transition from terrestrial to marine ~6–4 kya in response to the Holocene sea-level rise. Coastal salt pond cores record initial pond development as coral growth between island headlands cut off and isolated existing marine embayments ~2 kya. The decadal–centennial record is dominated by human activities, primarily road construction, manifested as a ~10× increase in sediment accumulation rate where development is heavy, a ~2.5× increase where development is moderate, and no detectable increase in undeveloped areas. Annual millimeter- to centimeter-scale laminae in salt pond cores represent individual depositional ‘events’. Light-colored, Ca- and Sr-rich sands represent marine overwash deposits that in some cases can be tied to tropical cyclones. Dark-colored Al-, Fe-, Ti-, Si-rich sandy muds represent island runoff when rainfall rates exceed a threshold of ~1.2 cm/day. Organic-rich layers represent microbial mat growth between depositional ‘events’. Marine overwash and rainfall/runoff layers fluctuate on an annual–decadal scale during the historical period, but show much more variability over the previous ~1400 years, suggesting sediment source(s), depositional processes, and/or driving mechanisms have not remained constant during at least the late Holocene.

Chronology of recent sedimentation and geochemical characteristics of sediments in Alvarado Lagoon, Veracruz (southwestern gulf of Mexico)

Coastal lagoons are very sensitive to anthropogenic impacts and sedimentary records may provide valuable temporal reconstructions of the environmental changes in the lagoon, the coastal zone, and the catchment area. The Alvarado Lagoon (Veracruz, southwestern gulf of Mexico) belongs to an extensive complex of wetlands recognized as a Ramsar site. However, its catchment basin has one of the highest deforestation rates in Mexico due to the transformation of lowlands for agriculture and grazing, thus causing siltation of the surrounding aquatic bodies. To evaluate the impact of land use change on Alvarado Lagoon, sediment fluxes and provenance were reconstructed by studying the elemental composition (determined by X-ray fluorescence spectrometry) of a sediment core dated radiometrically (210Pb, 137Cs, 239,240Pu). The sedimentary record showed an acceleration of the sediment accumulation rate during more than 134 ± 17 years; however, the changes in elemental composition during the past 40 years (i.e., 1972–2011) indicated the incorporation of continental weathered sediments and an increment of sediment accumulation of circa 470%, in coincidence with the period of higher deforestation and siltation. The recent increase in sediment accumulation rates and changes in geochemical features are attributed to erosion caused by land use changes in the Alvarado Lagoon drainage basin.

Modern rates of glacial sediment accumulation along a 15° S-N transect in fjords from the Antarctic Peninsula to southern Chile

[1] Rates of glacial erosion in temperate climates rank among the highest worldwide, and the sedimentary products of such erosion record climatic and tectonic signals in many glaciated settings, as well as temporal changes in glacier behavior. Glacial sediment yields are expected to decrease with increasing latitude because decreased temperature and meltwater production reduce glacial sliding, erosion, and sediment transfer; however, this expectation lacks a solid supportive database. Herein we present modern 210Pb-derived sediment accumulation rates on decadal to century time scales for 12 fjords spanning 15° of latitude from the Antarctic Peninsula to southern Chile and interpret the results in light of glacimarine sediment accumulation worldwide. 210Pb records from the Antarctic Peninsula show surprisingly steady sediment accumulation throughout the past century at rates of 1–7 mm yr−1, despite rapid warming and glacial retreat. Cores from the South Shetland Islands reveal accelerated sediment accumulation over the past few decades, likely due to changes in the thermal state of the glaciers in this region, which straddles the boundary between subpolar and temperate conditions. In Patagonia and Tierra del Fuego, sediment accumulates faster (11–24 mm yr−1), and previously collected seismic profiles show that rates reach meters per year close to the glacier termini. This increase in sediment accumulation rates with decreasing latitude reflects the gradient from subpolar to temperate climates and is consistent with glacial erosion being much faster in the temperate climate of southern Chile than in the polar climate of the Antarctic Peninsula.

Paleomagnetic and fission-track dating of a Late Cenozoic red earth section in the Liupan Shan and associated tectonic implications

The north-trending Liupan Shan (六盘山) is an important tectonic boundary between the Tibetan Plateau and the Ordos platform. The Late Cenozoic red earth deposits of the Liupan Shan record its tectonic history and environmental effects. In this article we report a new Late Cenozoic red earth section from an intermontane basin in the southern part of the Liupan Shan. Lithofacies analysis, paleomagnetic and fission-track chronologies, and paleocurrent analysis have been employed to identify the tectonic uplift events of the Liupan Shan. Based on the age constraints of mammal fossils, the paleomagnetic polarity zones of the Huating (华亭) Section can be approximately correlated with the standard polarity zones that lie between C3An.2n and C5n.1n of the Geomagnetic Polarity Timescale; the bottom age of this section is approximately 10 Ma. Based on this and the previous studies, we infer that a tectonic event commenced in the southern Liupan Shan in this interval between 8.3 and 8.7 Ma, accompanied by a remarkable increase in sediment accumulation rate. Field observations, fission-track dating, determinations of grain-size frequency distributions and the vertebrate fossils found there suggest that the red earth deposits were reworked by water and mainly transported by fluvial-alluvial processes from the adjacent area.