High Sediment Accumulation Rates Research Articles

Burial of Organic Carbon in Swedish Fjord Sediments: Highlighting the Importance of Sediment Accumulation Rate in Relation to Fjord Redox Conditions

AbstractFjords are net carbon sinks with high organic carbon (OC) burial rates; however, the key drivers of OC burial in these systems are not well constrained. To study the role of water column redox condition and OC composition on OC preservation in fjord sediments, we determined OC accumulation rates (OCAR), OC source, and OC degradation in three Swedish fjords with variable redox conditions (long‐term oxic, seasonally hypoxic, and long‐term anoxic). Average OCARs were variable between and within the fjords studied (2–122 g OC m−2 yr−1), but highest rates were at the mouth for each fjord. Based on a δ13C mixing model, Swedish fjords bury predominantly marine‐derived OC (∼83% of the total OC burial) likely because of relatively gentle slopes, low riverine discharge, and high marine inflow. Using a multi‐biomarker approach (lignin, photosynthetic pigments, and total hydrolyzable amino acids) we found, terrestrially‐ and marine‐derived OC were moderately degraded under the various redox conditions sampled, suggesting water column redox and OC source are not primary drivers of OC burial in these fjords. Rather, high sediment accumulation rates, common to fjords globally, lead to low oxygen exposure times, thus promoting efficient burial of OC regardless of its chemical composition.

The sedimentology of gravel beds in groundwater‐dominated chalk streams: Implications for sediment modelling and management

AbstractElevated fine sediment accumulation in a river system's gravel bed is known to cause detrimental ecological impacts. Current sediment targets and approaches to mitigation have failed due to the oversimplification of geomorphological processes controlling fine sediment accumulation and the lack of relevant scientific knowledge underpinning them. This is particularly apparent in chalk streams (groundwater‐dominated systems) which regularly exhibit high rates of sediment accumulation despite low suspended sediment yields. A necessary first step is to better characterise their sedimentology; thus, the novelty of this study was to determine the sedimentological characteristics of chalk stream gravel beds, specifically the quantity and distribution of fine sediment with depth. We collated published and unpublished freeze‐core data, encompassing 90 sites across 11 UK chalk streams. Results showed average quantities of fine sediment (<2 mm) in chalk stream gravel beds were 25% by weight, with >75% of beds exceeding thresholds for ecological degradation. Quantities of fine sediment increased with increasing depth into the bed, with an average increase between surface and subsurface layers of 54%, and 89% of the gravel bed over‐saturated with fine sediment. Regional differences were attributed to differences in stream power and local sediment sources, including surficial geology and catchment land use. Additionally, a major contrast was identified between experimental conditions in flume studies used to establish models describing interactions/mechanisms of fine sediment infiltration into immobile gravel beds and the natural conditions observed in chalk streams. As such, the use of such models as a basis to explore sediment management scenarios is unlikely to predict the outcome of such management techniques correctly in a real‐world situation.

Near seafloor methane flux in the world's largest human-induced dead zone is regulated by sediment accumulation rate

The vast oxygen-depleted area of the central Baltic Sea is the largest human-induced dead zone in the world with 70,000 km2 or approximately three times the second largest one in the Gulf of Mexico. Methane occurs in high concentrations in bottom waters (3200 nM) and sediments (30 mM), and its dynamics is better constrained for the water column, but still poorly understood on sediments. Here we show that sediment accumulation rate plays a major role in regulating the quantity of organic matter and its residence time in the sulphate reduction and methanogenesis zones and, therefore, affects methane generation, consumption, and diffusive flux in sediments near the seafloor (< 1 m). High fluxes found in high sediment accumulation rate areas and competition for substrate (organoclastic sulphate reduction vs. anaerobic oxidation of methane with sulphate), compromise the ability of the thin microbial filter to consume and prevent methane diffusion through the seafloor.

Multi-annual and multi-decadal evolution of sediment accretion in a saltmarsh of the French Atlantic coast: Implications for carbon sequestration

Coastal marshes offer natural solutions for adapting to and mitigating the effects of climate change and sea level rise. However, the resilience of the marsh physical system and, with it, the ecosystem services that it provides, is largely site specific. This calls for the increase in the spatial cover of coastal marsh studies in order to assess the controlling factors of marsh evolution, and their long-term carbon storage capacities. Here, we study the spatio-temporal variations in sedimentation rates and organic carbon (OC) sequestration capacity of the macrotidal minerogenic saltmarshes in Aiguillon Bay, belonging to one of the largest French coastal marshes. Supported by aerial photographs and satellite image analysis, we first show that saltmarshes of the Aiguillon Bay have prograded at very high rates, up to 14 m yr−1 since 1950. Sediment accumulation rates (SAR) were estimated at both multi-annual to multi-decadal scales based on two approaches: (i) LiDAR-based digital elevation models from multiple acquisition dates (2010–2021); and (ii) depth profiles of 210Pb in excess and 137Cs in sediment cores collected along cross-shore transects in the saltmarshes. Long-term SAR range from 0.8 to 2.2 cm yr−1 and are among the highest reported worldwide for equivalent systems. The positive accretion balance (accretion rate minus local sea-level rise rate) provides important clues on marsh resilience suggesting that the Aiguillon Bay is currently able to adapt to rising sea level. Despite relatively low organic carbon content (1.3–6.0%), high SAR leads to high carbon sequestration rates (99–345 gC m−2 yr−1; or a mean value of 2.5 Mg C ha−1 yr−1). The isotopic signature of sediment OC reveals a significant and rapid decomposition of organic material in surface cores, while allochthonous sediment of marine origin dominates the signature of chemically-stable OC of marsh sediments. This implies that the carbon sequestration capacity of minerogenic saltmarshes, such as those of the Pertuis Charentais, also depends upon the wealth of adjacent coastal environments through high sediment supply and primary productivity.

Porewater and solid phase speciation geochemistry of phosphorus in the Western Bay of Bengal: An assessment of depositional pathways

In this study, we used a combination of porewater phosphate and solid-phase P speciation to examine the present-day benthic biogeochemical cycling and pathways of phosphorus (P) deposition at four sites in the western Bay of Bengal (BoB). We concentrate on two regions, with 2 sediment cores from each region. One is the Palar basin where ancient phosphorites and phosphatic grains were discovered before, and the other in the Cauvery basin, where no phosphories are known to occur. Both of these regions fall in the depths of the oxygen minimum zone (OMZ). Our results suggest that the influx of lithogenic material, or lack of it, largely regulates the sedimentary cycling of P in BoB. Both locations lack active phosphogenesis due to substantial inputs of non-reactive P relative to reactive P from river runoff and its quick burial by the ballasting effect, which appears to prevent P from transforming into an authigenic phase. Despite the oxygen-depleted conditions, low bacterial respiration limits the microbial reduction of iron oxyhydroxide and makes the redox cycling of iron a weak player in the sedimentary cycling of P in the studied cores. The burial efficiency of P is relatively high in the Cauvery basin cores owing to the high sediment accumulation rate and suggests that most of the sedimentary P is undergoing burial with a minimum regeneration. Lack of/Very low recent sedimentation in the Palar basin is possibly due to the reported migration of river course towards the south during the mid-to Late Holocene.

Tectonic and climate forcing of exhumation in the SE Tibetan Plateau over the past 7 Ma: Insights from the deltaic-submarine fan system in the Andaman Sea, northeastern Indian Ocean

The Tibetan Plateau and its adjacent areas are a typical example for studying the relationship between plateau uplift, climate change, and surface erosion. It has been argued that the South Asian monsoon (SAM) strengthened significantly at 10–8 Ma, whereas others suggested that the SAM weakened at that time. Here we report a large deltaic-submarine fan system in the Andaman Sea with a high sediment accumulation rate during the Pliocene. We propose that the formation of the deltaic-submarine fan was a result of climate and tectonics over the past 7 Myr. Our findings are based on multi-method provenance analyses (heavy minerals, bulk-rock geochemistry, detrital zircon UPb ages, and seismic data) of the Late Miocene-Pleistocene deltaic-submarine fan system. The analyses, in combination with those of the modern river sediments and potential source regions, suggest that the Upper Miocene sediments were mainly derived from the eastern Lhasa Terrane and northern West Burma Terrane (Upper Salween and Upper Irrawaddy, respectively), whereas the Pliocene-Pleistocene deposits were primarily derived from the eastern Lhasa Terrane, northern West Burma Terrane, and Tengchong Terrane (Lower Salween River), with small inputs from the Sibumasu and Baoshan terranes. We propose a modified scenario in which the Salween initiated in the early Late Miocene (10 Ma) and the Irrawaddy developed a modern geometrical morphology after the Pliocene. These two large rivers transported eroded materials from the SE Tibetan Plateau into the Andaman Sea and formed the deltaic-submarine fan. The high sediment accumulation rate (>530 m/Myr) in the Pliocene is consistent with the strengthened SAM and heterogeneous crustal deformation of the SE Tibetan Plateau during the same interval. We conclude that both the climate and tectonics have direct effects on exhumation of the SE Tibetan Plateau over the past Late Miocene-Quaternary.

Assessment and projections of sediment budget resilience in Marbella, Spain

This paper discusses how the beaches of Marbella, on the southwestern Mediterranean coast of Spain, may no longer stay in their natural resilience envelope due to a critically delicate natural sediment transport balance and a degree of artificialization that has entered the whole sedimentary system into a new resilience state. The combination of vigorous terrain and millenary human action and disruptions across and alongshore on the coastline have increased stress on sediment availability. Although sediment circulation in the coastal cell has often been studied, the investigation of the connection between soil loss and river sediment transport and retention at a major dam remains a challenge. In this article, a first-order sediment yield prediction was established by using a GIS-based model applied to the area’s main river basin, and validation of model results is provided by empirical measurements of sedimentation in the main reservoir lake of La Concepción using Differential Global Positioning System (D_GPS)/Echo-sounder combination and measurements from remotely piloted aircraft compared with preconstruction blueprint topography documenting spot heights where sediments accumulated or eroded over 50 years. The marine circulation is interpreted from previous research by the authors that established a source-sink pattern similar to the Atlantic platform-fed marine system that originated the significant Cabopino dunes. The significant erodibility that we have estimated seems matched by potentially high sediment accumulation rates along selected profiles and spot heights across the bottom of the reservoir lake, and, in combination with the marine circulation model, our results identify that sediment budget key elements in Mediterranean settings, such as soil loss, sediment entrapment in reservoirs, and the coastal marine circulation, are in a state of deficit that suggests that the resilience envelope is surpassed and the system as a whole is entering a new resilience state in which the engineering factor is key. Some ecosystem services, such as the protection offered by the natural resilience of the beach and dune system of Cabopino, are no longer recoverable in the current artificialization state of the system as a whole.

Actinium and radium fluxes from the seabed in the northeast Pacific Basin

Five sediment cores were collected along a cruise tract from Hawaii to Alaska in August 2017 (C-Disk-IV cruise) with the objective of characterizing the behavior of 227Ac, 228Ra, and 226Ra and their fluxes into the overlying water column, information that is essential to the interpretation of the distribution of these tracers in the ocean, for example, as measured on GEOTRACES cruises. Solid phase profiles of these isotopes were measured, and reaction-transport models were applied that incorporated molecular diffusion, bioturbation, sedimentation, distribution coefficients (kd), and the fraction of each isotope released to pore water by parent decay (called F). Fits to these profiles used kd values determined in lab experiments for C-Disk-IV sediments. Ra kd values (1000–3000 mL g−1) agreed with previous estimates for deep-sea sediments, and Ac kd values (3500–22,000 mL g−1) correlated with those for Ra but were about 7 times greater.Two independent approaches were used to quantify the benthic fluxes of 227Ac and 228Ra in the Northeast Pacific: (1) use of solid phase profiles with a reaction-transport model, as well as integrated downcore daughter-parent deficiency; and (2) direct measurement of fluxes based on core incubation. The two independent methods agreed within uncertainty, and the average 227Ac and 228Ra sediment fluxes for the Northeast Pacific are 90 ± 20 and 600 ± 200 dpm m−2-yr−1, respectively. The 226Ra sediment flux was only determined by the former approach, and the flux calculated in this study is similar to previous work in the North Pacific, averaging 1300 ± 200 dpm m−2-yr−1. This is over 2× higher than the water column inventory of 226Ra in this region (600 dpm m−2-yr−1), and indicates the importance of lateral 226Ra export from the N. Pacific.The largest 227Ac and Ra isotope fluxes in the study area are near the center of the Northeast Pacific (∼37°N). Smaller 227Ac, 228Ra and 226Ra fluxes occur north of 40°N, primarily due to dilution of their Pa and Th ancestors by higher sediment accumulation rates.

High-resolution record of geochemical, vegetational and molluscan shifts in a Central European spring-fed fen: implications for regional paleoclimate during the early and mid-Holocene

Despite the increasing interest in spring-fed fens’ sediments as a palaeoenvironmental archive, their potential as high-precision climatic records is rarely used to its full extent. Here, we present a detailed early to mid-Holocene record of environmental changes in the Turtul hanging spring-fed fen, northeastern Poland, to test whether the well preserved and accurately subsampled alternating peat and tufa sediments allow us to recognise short-duration local and regional climatic changes. Our reconstructions are based on a loss-on-ignition analysis of the sediment, carbon and oxygen stable isotope values of tufa, and biotic proxies: plant macrofossil and malacological analyses. A detailed environmental reconstruction was possible due to the combination of detailed sampling (1 cm intervals) and high sediment accumulation rates. The two sediment sequences collected from the fen revealed the congruent palaeoenvironmental history; however, the temporal shift associated with the distance of the coring site from the water outflow was also apparent, stressing the need for careful selection of the coring site in palaeoenvironmental research. Peat deposition at Turtul started at 10,300 cal yr BP; however, tufa precipitation did not begin before 9250 cal yr BP. During the most active tufa deposition, from 7850 to 6100 cal yr BP, a high sedimentation rate was observed (0.11–0.25 cm yr−1). We found several abrupt and short-duration declines in carbonate deposition corresponding to climatic fluctuations recorded in geological archives from the southeastern Baltic region. The carbonate drops at 8200–8130, 7900–7850, 7400–7300, 6600–6500, 5980–5870 and 5400 cal yr BP were associated with climate cooling or drying, whereas those at 10,200–9250, 8450–8340 and 6160–6100 cal yr BP can be attributed to drying with no cooling. We conclude that the evident sensitivity of the Turtul fen to climatic shifts that emerge from our high-resolution (one sample every 17 years on average) reconstruction complements the still rare high-resolution palaeoenvironmental records in the area.

Lipid biomarkers in Lake Wudalianchi record abrupt environmental changes from the volcanic eruption in 1776

Volcanic eruptions can cause abrupt changes in surrounding environments, but detailed sediment records of such changes are rare. The very high sediment accumulation rate in Lake Wudalianchi, a volcanic-dammed freshwater lake in northeastern China, provides an excellent opportunity for reconstructing volcanic-induced environmental change. Here we use multiple lipid biomarkers to reconstruct changes in aquatic ecosystems, hydrology, and methane cycling before and after a regional explosive volcanic eruption in 1776. Our results indicate that the Wudalianchi catchment was a shallow organic-rich marsh environment with abundant aquatic emergent macrophytes and an intense microbial methane cycle before the volcanic eruption. Following the volcanic eruption, the marsh was rapidly transformed into an oligohaline lake environment due to river channel damming by lava. Associated with the abrupt environmental change, we observe corresponding changes of cyclisation ratios of branched glycerol dialkyl glycerol tetraethers or brGDGTs (CBT and CBT′ indices), and the inferred pH displays an abrupt increase of ∼1.3. Group 1 alkenones emerge abruptly after the eruption when the inferred pH exceeds 8. Our results provide the first high-resolution reconstruction of environmental change induced by volcanic eruption and demonstrate the high efficacy of the lipid biomarkers in recording aquatic and terrestrial changes.

Chronostratigraphic and palaeogeographic interpretation of Nakdong deltaic sequences in the south-eastern Korean Peninsula

We investigated the chronostratigraphy of six boreholes in the Nakdong River delta area in South Korea based on optically stimulated luminescence (OSL) and 14C dating. The standardised growth curves (SGCs) produced from single and multiple boreholes performed well in determining low equivalent dose (De; < 40 Gy). 14C dates from shell and wood fragments were mostly representative of depositional time. Based on 14C and OSL ages, the lowermost fluvial sediments showed relatively low sediment accumulation rates (0.1–0.8 m/kyr−1) prior to 12 ka. A hiatal discontinuity ranging from 12 to a maximum of 50 ka resulted in a break in sedimentation between the Last Glacial Maximum and the Holocene. A thick fluvio-estuarine unit in the lower part indicated relatively high accumulation rates (3.3–9.4 m/kyr−1) from 12 to 9 ka, which corresponded to rapid sea level rise. In the middle part, decreased accumulation rates (1.7–5.5 m/kyr−1) from approximately 9 to 6 ka were interpreted as transgressive deepening. Lower accumulation rates (0.9–3.2 m/kyr−1) during the middle to late Holocene were caused by relative sea level fall and a horizontally extended sedimentation area. In the upper part, the highest sediment accumulation rates (3.5–8.3 m/kyr−1) and rapid delta progradation during the last 2.0 ka might have been caused by widespread human influence and monsoonal climatic change. Isochrons of 1-kyr scale were identified based on the lithologic boundary and age results for each borehole, which represent palaeoshoreline positions during the transgressive cycle and boundaries of considerable delta progradation.

Asbestos mining waste impacts on the sedimentological evolution of the Bécancour chain of lakes, southern Quebec (Canada)

More than a century (1877–2011 CE) of asbestos mining activities in the Thetford Mines region have resulted in the accumulation of gigantic mineral waste piles on the banks of the Bécancour River (southern Quebec, Canada). This river widens downstream from the mining sites to form a chain of lakes, successively: Stater Pond, Trout Lake, Lake William and Lake Joseph. A previous paleolimnological investigation revealed that waste erosion and transport strongly modified and polluted Trout Lake. However, questions remain about the extent of the mining contamination within the Bécancour River Basin and its impacts at other spatial scales. Here, we aimed to address this lack of knowledge by analyzing the sedimentological evolution of Stater Pond and lakes William, Joseph and Bécancour (upstream reference site). Radiometric dating (210Pb, 137Cs, 14C) and analyses of geochemical composition (ICP-AES/ICP-MS), computed tomography, magnetic susceptibility, loss-on-ignition and grain size were performed on sediment cores retrieved at these sites. In contrast to Lake Bécancour, yet similar to Trout Lake, we found that Stater Pond and Lake William have received high mineral matter loads since the creation of the Lake Asbestos Mine during the 1950s. Recent lake sediments at these downstream sites were highly enriched in magnesium, chromium and nickel. Comparison of their geochemical signature with that of sedimentary source materials from within the drainage basin demonstrated that they predominantly originate from mining waste erosion. Because of this issue, Stater Pond and lakes Trout and William are nowadays exposed to very high sediment accumulation rates (up to 1.4 cm yr−1; 0.6 g cm−2 yr−1) and heavy metal enrichment. Evidence for contamination was also found in Lake Joseph, indicating that wastes are transported and deposited over ≥25 km downstream from the mining sites. Our study highlights the high risks and dangers associated with asbestos pollutants in aquatic ecosystems.

A 2300-year record of glacier fluctuations at Skilak and Eklutna Lakes, south-central Alaska

Holocene glacier fluctuations in south-central Alaska reflect hydroclimate changes in a region strongly influenced by large-scale features of North Pacific climate. Glacier fluctuations over the past approximately 2300 years were inferred from multiple geophysical and geochemical properties of a new 13-m-long sediment core from proglacial Skilak Lake. The chronology is based on annual-layer (varve) counting, supported by radiocarbon ages and the identification of the 1912 CE Katmai tephra. Sediment density and accumulation rate show two prominent century-scale increases, one near the base of the cored sequence (around 270-370 BCE) and one near the top (around 1600–1900 CE, Little Ice Age). These two peaks, and the absence of such increases between them, coincide with sedimentation-rate changes as previously measured in multiple cores from Eklutna Lake, located 130 km northeast of Skilak Lake. The similarity between the two sedimentary sequences, despite their contrasting geomorphic settings and spatial scales, implies that regional climate change is the overriding control. Sediment with high bulk density and thick varves was deposited concurrently with the expansion of glaciers in the catchment and elsewhere in the region during the Little Ice Age. We likewise attribute the deposits with similar characteristics to expanded glaciers centered around 320 BCE, and the absence of such sediment during the intervening 1800 years to the lack of a major glacier advance. Climatologically, both periods of high sediment accumulation rate correspond with times of intensified Aleutian Low pressure, as recently reconstructed based on regional syntheses of oxygen-isotope records for the earlier period, and on sea-salt concentration in two Alaskan ice cores for the later period. Both periods also coincide with pronounced multi-decadal downturns in combined solar and volcanic global radiative forcing. The massive buildup of glaciers during the Little Ice Age might reflect the increased frequency of these downturns combined with millennial-scale decrease in orbitally driven insolation. Glaciolacustrine records from these two lakes add a new perspective on the multi-century variability of glaciers and hydroclimate in the North Pacific region.

Foraminifera-derived carbon contribution to sedimentary inorganic carbon pool: A case study from three Norwegian fjords.

Norwegian fjords have been recently recognized as hot spots for carbon burial due to the large amounts of terrestrial organic matter delivered to fjord sediments, as well as the high sediment accumulation rates. Here, we present the first data on the contribution of benthic foraminiferal inorganic carbon to the sediments of three Norwegian fjords. Our study shows that calcareous foraminifera, which are among the most abundant calcifying organisms in the modern global oceans, can constitute between 15% and 33% of inorganic carbon accumulated in the sediments of the two studied southern Norwegian fjords (Raunefjorden and Hjeltefjorden). In a northern Norwegian fjord (Balsfjorden), the contribution of calcareous foraminifera to the inorganic carbon pool is smaller (<1%) than the one observed in southern fjords. We also found that the amount of foraminifera-derived carbon is primarily dependent on the species composition of the foraminifera community. Large calcareous foraminifera species, despite a lower number of individuals, constitute, on average, 13%-29% of the inorganic carbon in the two southern Norwegian fjords, while the contribution of small, highly abundant species does not exceed 4% of the inorganic carbon pools in the sediments. Calcareous foraminifera species that are indicative of dysoxic conditions have been found to have low inorganic carbon contents per specimen compared to other analysed similar-sized calcareous foraminifera species. This relationship most likely exists due to the thin test walls of these foraminifera species, which may facilitate gas exchange. The results of our case study suggest that the climate-driven formation of near-bottom low-oxygen zones may lead to the dominance of foraminifera associated with dysoxic conditions and, in consequence, to the decrease of foraminifera-derived inorganic carbon. However, to properly analyse the contribution of carbon from thin-walled foraminifera to the sedimentary carbon pool, further studies analysing a broader range of these species is needed.

Vegetation Determines Lake Sediment Carbon Accumulation during Holocene in the Forest–Steppe Ecotone in Northern China

To understand the past carbon accumulation of forest–steppe ecotone and to identify the main drivers of the long-term carbon dynamics, we selected Huangqihai Lake and analyzed the sediment records. We measured the organic carbon content (TOC; %) of sedimentary samples and quantified the carbon accumulation rate (CAR; g C m−2 yr−1). Furthermore, the climate, soil erosion, and vegetation development of the past 6800 years were reconstructed using physicochemical parameters and pollen records. Human activities were also obtained from a 2200-year history record. Our results showed that the CAR was high during 5800~4100 cal yr BP (40~60 g C m−2 yr−1), which is mainly attributed to the high sediment accumulation rate (SAR) during this period. Pearson’s correlation, redundancy analysis and hierarchical variation partitioning analyses suggested that the CAR was influenced by the SAR and TOC, while vegetation dynamics (broadleaved tree percentage and vegetation coverage) and local soil erosion were the main drivers of the TOC and SAR. Especially when the vegetation was dominated by broadleaved forests, the CAR was significantly high due to the high gross primary productivity and carbon density of forest compared with steppe. Our study highlights the direct influence of local vegetation and soil erosion on the CAR, whereas climate might influence indirectly by changing local vegetation and soil conditions. Moreover, our results showed that human activities had positive influences on the carbon accumulation dynamics in this region since 2200 cal yr BP by influencing the SAR.

Grain-surface microtextures in deposits affected by periglacial conditions (Abalakh High-Accumulation Plain, Central Yakutia, Russia)

Our paper describes and interprets grain microtexture and microstructure collected from periglacial sediments on the Abalakh High-Accumulation Plain (AHAP) in Central Yakutia. This territory occupies the Lena–Amga Rivers interfluve. In borehole 18/1, five sediment Complexes (I–V) of successive environments were recognized: 1) alluvial in the base of the borehole—Complex I; 2) alluvial-lake—Complex II; 3) lake-complex—Complex III; 4) ice-complex (yedoma)—Complex IV; and finally 5) a Holocene cover—Complex V. Quartz sand-grain and silt-grain microtextural analysis was undertaken in a scanning electron microscope (SEM) and supplemented by mineralogical analyses to reconstruct the sedimentary-accumulation environment, discern the influence of periglacial conditions on the grains, and identify the sediment source(s) for each complex. Based on the results, a conclusion can be reached that the accumulation of Complex I took place as a result of multiple repetitive transportation events recycling the same material and introducing a limited supply of new material into the fluvial environment. Upward in the succession, fluvial-process activities decreased in favour of lake-deposit accumulation. Frozen syngenetic ice-rich silty deposits—yedoma or ice complex—of Complex IV are composed of grains with a precipitated surface, but differ from the underlying deposits in the degree of crusting and mineralogy. Most probably aeolian processes are responsible for their transport. They include a variety of sediments, including older-sourced sediments such as retransported loess and the detritus from mechanical weathering coeval with sediment accumulation. Traces of frost and chemical weathering have been identified on the grain surfaces, the former visible in the form of breakage blocks and conchoidal fracture microtextures and the latter – as surface crusting. However, the frequencies of these microtextures are low, which suggests a relatively high rate of sediment accumulation.

Evolution of the Thermal and Dehydration State of Sediments Entering the North Sumatra Subduction Zone

AbstractShallow slip on the plate‐boundary fault during the 2004 Mw 9.2 Aceh‐Andaman Earthquake, offshore North Sumatra, has been linked to thick incoming sediments on the oceanic plate with advanced diagenetic and sediment property changes at the depths of plate boundary fault development. We couple age control, physical, and thermal property measurements from International Ocean Discovery Program (IODP) drilling with multichannel seismic reflection data to reconstruct the thermal structure and evolution of the incoming sediment column, up to the point of accretion/subduction. Lithospheric thermal rejuvenation around 58 Ma is required to explain anomalously high heat flux at IODP Site U1480, and heat flux within the trench is suppressed by very high sediment accumulation rates during the development of a thick trench wedge. Accumulation of up to ∼4.5 km of thick Nicobar Fan and trench wedge sediments produces temperatures &gt;150°C within the basal sediments where the décollement develops, resulting in total pre‐subduction diagenetic dehydration of basal sediments. The smectite‐illite transformation within these basal sediments produces sufficient fluid to explain a polarity reversal on a pre‐décollement reflector. We suggest that the boundary between basal‐pelagic and siliciclastic‐fan sediments has trapped fluid over the last ∼1 Myr, as a result of primary lithological properties, diagenetic fluid release, and cementation, controlling décollement formation at this weakened level. Pre‐subduction dehydration of large portions of the accreted sediment column strengthens the décollement beneath the prism, extending co‐seismic velocity‐weakening behavior close to the trench, which may occur at other subduction zones with similar sediment compositions, thicknesses, and/or temperatures.

Stratigraphy, age, and provenance of the Eocene Chumstick basin, Washington Cascades; implications for paleogeography, regional tectonics, and development of strike-slip basins

Strike-slip faults form in a wide variety of tectonic settings and are a first-order control on the geometry and sediment accumulation patterns in adjacent sedimentary basins. Although the structural and depositional architecture of strike-slip basins is well documented, few studies of strike-slip basins have integrated depositional age, lithofacies, and provenance control within this context. The Chumstick basin formed in central Washington during a regional phase of dextral, strike-slip faulting and episodic magmatism associated with Paleogene ridge-trench interaction along the North America margin. The basin is bounded and subdivided by major strike-slip faults that were active during deposition of the intra-basinal, non-marine Chumstick Formation. We build on the existing stratigraphy and present new, detailed lithofacies mapping, conglomerate clast counts (N = 16; n = 1429), and sandstone detrital zircon analyses (N = 16; n = 1360) from the Chumstick Formation to document changes in sediment provenance, routing, and deposition. These data allow us to reconstruct regional Eocene paleo-drainage systems of Washington and Oregon and suggest that drainage within the Chumstick basin fed a regional river system that flowed to a forearc or marginal basin on the newly accreted Siletzia terrane. More generally, excellent age control from five interbedded tuffs and high sediment accumulation rates allow us to track the evolving sedimentary system over the Formation’s ca. 4−5 m.y. depositional history. This is the first time lithofacies and provenance variations can be constrained at high temporal resolution (0.5−1.5 m.y. scale) for an ancient strike-slip basin and permits a detailed reconstruction of sediment routing pathways and depositional environments. As a result, we can assess how varying sediment supply and accommodation space affects the depositional architecture during strike-slip basin evolution.

Modelling Ediacaran metazoan–microbial reef growth

AbstractThroughout the Phanerozoic, sessile metazoans grew in close association with various microbial carbonates to form reefs. The first metazoans with calcareous hard‐parts appeared in the terminal Ediacaran, ca 550 million years ago, and these also commonly grew associated with microbial mats, thrombolites and stromatolites, to form the oldest known metazoan–microbial reefs. These hard‐parts also formed the first skeletal (bioclastic) carbonate sediments, which increased to dominate shallow marine carbonate sedimentary production during the Phanerozoic. Here the growth dynamics and sedimentary interactions between Ediacaran microbial–metazoan reefs and their bioclastic products are described based on reef complexes from the Nama Group, Namibia (ca 547 Ma), and the first three‐dimensional numerical models are constructed to parametrize these dynamics. These reefs are observed to form large domal mounds and columns neighbouring locally high rates of bioclastic sediment accumulation, and commonly evolve to flat‐topped surfaces overlain by sediment. This model is parsimonious, embodying a single dynamic rule: metazoans attach to microbialite mounds which grow radially into free space shedding bioclasts in proportion to metazoan production. Models evolve mounds into columns due to spatial competition, and produce an oscillating interplay between mound expansion and smothering that results in an inter‐fingering of microbialite mound and detrital sediment. This dynamic is shown to be non‐linear in the proportion of bioclastic sediment produced. Smothering by bioclastic sediment is also demonstrated to both reduce the rate (by volume) of subsequent mound growth, and to overwhelm the growing mat surfaces of stromatolites: after a threshold is reached in the rate of bioclastic sediment deposition, sediment terminates all stromatolite growth. These models show that the general characteristics of field observations can be explained by a single dynamic rule, and that reef‐sourced bioclasts may make an important local contribution to Ediacaran to Cambrian microbial–metazoan reef dynamics – a contribution that remains important throughout the Phanerozoic.

Environmental controls over Holocene carbon accumulation in Distichia muscoides-dominated peatlands in the eastern Andes of Colombia

The long-term carbon (C) and vegetation dynamics of tropical, high-Andean cushion peatlands are poorly understood. Here, we present radiocarbon-dated paleoecological records and modern microclimate data from high-elevation peatlands currently dominated by the cushion plant Distichia muscoides in the páramo (alpine tundra) of the eastern Colombian Andes. Focusing on a well-dated 4300-year-old peat core collected at 4194 m elevation from the Sierra Nevada del Cocuy, we investigated the response of peatland vegetation to hydroclimate changes and environmental controls on C accumulation. In addition, we carried out a regional synthesis of existing C accumulation data from northern Andean peatlands to place our new results in broad geographic and temporal context over the last 17,000 years. Our results show that the current Distichia-dominated peatlands are a recent landscape feature on El Cocuy occurring only during the last ∼250 years. The Distichia peatlands have extremely high sediment accumulation rates of up to 1 cm yr−1 and C accumulation rates between 183.7 and 849.5 g C m−2 yr−1, decreasing with the elevation of a site. These rates far exceed an expected modern C accumulation rate of 110 g C m−2 yr−1, calculated by extrapolating an exponential function from a late Holocene average C accumulation rate of 48.6 g C m−2 yr−1 for the northern Andes. This suggests that recent Distichia establishment represents an acceleration of C accumulation at a rate higher than expected from decomposition-related autogenic effects. Before the modern climate warming period, our record indicates that Distichia cushions have only grown intermittently, often after the peatland was flooded with mineral sediments. Mineral sediment deposits at 3290, 2590, 1620, 930, and 860 cal. yr BP correspond with wet intervals from lake-derived records of hydroclimate change in the northern Andes and indicate increases in precipitation and subsequent in-wash of mineral sediments. However, within the last 150 years, mineral sediment deposition has increased in frequency, likely due to warming-induced melting of permanent snow and ice mobilizing exposed fluvio-glacial sediments. Increased mineral nutrients, rising temperatures, and strong diurnal temperature contrast in the tropical mountains may explain the rapid C accumulation rates. Taken together, we show that Andean peatlands have experienced the largest shift in vegetation and C accumulation rates in the last 250 years than at any other time during the last 4000 years.