Long-term Sediment Accumulation Research Articles

Spatial and temporal variation in long-term sediment accumulation in a back-barrier salt marsh

In situ persistence of salt marshes in the face of sea-level rise relies on their ability to maintain substrate elevation through sufficient vertical accretion of sediment. However, sedimentation rates in salt marshes vary spatially and temporally, which complicates the assessment of their ability to keep up with sea-level rise. Here, we explore the spatial and temporal variation in sediment accumulation in a single back-barrier salt marsh site. Using one-time in situ measurements at the landscape scale, we obtained synoptic information of elevation and sediment thickness over the entire salt marsh in a chronosequence over centuries. Repeated measurements along short elevation transects (0.3–0.9 m +MHT) revealed decadal changes, complementing the broader marsh data with detailed information on elevation, thickness of the marsh deposits and accumulation rates. Thickness of the deposits was largely related to the elevation gradient: the sediment layer was thinner at the higher marsh (near the dunes and far away from the intertidal flats), and thicker at the lower marsh (near the intertidal flats). Moreover, the thickness of the layer increased with salt marsh age along the chronosequence, and age accounted for 72 % of variability in sediment accumulation. The rate of sediment accumulation was higher than the local rate of sea-level rise in the younger marsh, whereas it was equal to the rate of sea-level rise in the older marsh. In the older salt marsh, sediment accumulation was lower, possibly due to autocompaction in the thicker, older layers. Both at the landscape scale and along short elevation transects within individual drainage basins, sediment accumulation decreased with distance to sediment supply routes. However, their relative importance depended on the scale of observation. Distance to creeks accounted for 17 % of the variability in sediment accumulation at the landscape scale, compared to 4 % at the smaller scale. Similarly, the influence of distance to intertidal flats varied from 1 % at the landscape scale to 13 % at the smaller scale. Our main findings indicate that lower-elevation older marshes and higher-elevation younger marshes far away from sediment sources are at risk of not keeping pace with the local rate of sea-level rise and are potentially vulnerable to increased flooding.

System-scale dynamics of long-term sediment and phosphorus accumulation in contrasting valley-bottom wetlands in dryland landscapes of South Africa

Anthropogenic-induced acceleration of sediment and associated nutrients presents a serious challenge to water resource management across the globe. While wetlands are potential natural solutions, little is known of how well key wetland types attenuate downstream sediment fluxes and how these relate to local- and catchment-scale factors. This research aimed to develop an understanding of patterns and rates of sediment and phosphorus retention in valley-bottom wetlands in South Africa. The geomorphology, sedimentology and historical accretion rates of three wetlands which offered diversity in size, catchment position and degree of organic accumulation were compared to ascertain the impact of local- and catchment-scale factors. Estimates of sediment and associated phosphorus accumulation suggest higher rates in clastic sediment-dominated wetlands (1089–7655 g sediment m−2 yr−1, 0.4–1.6 g-P m−2 yr−1) compared to an organic sediment-dominated system (601–1308 g-sediment m−2 yr−1, 0.2–0.3 g-P m−2 yr−1). Sediment accretion rates generally increased longitudinally, indicating that distal wetland reaches act as sediment and phosphorus retention hotspots. Phosphorus distribution patterns were primarily attributed to variations in organic matter content and associated aluminium and iron complexing agents in fine-grained sediment deposits, while particle size distributions were less important. However, the relevance of individual complexing agents in phosphorus dynamics varied among wetlands with differing landscape conditions. Contrasts between these systems reflect variations in catchment lithology and hydroclimatic regime, which influence the relative magnitude of sediment supply and vegetation productivity. We postulate that intermediate-sized wetlands characterised by catchments around 900 ha in size, gentle longitudinal slopes (∼2 %), a high proportion of fine sediment deposition, limited organic matter breakdown, and a long vegetation growing season provide optimal conditions for sediment and phosphorus accumulation. This system-scale information is essential to guide management decisions to identify and prioritize within-catchment ‘hotspots’ of sediment and phosphorus retention.

Patterns and processes of channel and floodout adjustment in a discontinuous dryland river, semi-arid eastern Australia

Alluvial rivers in semi-arid and arid landscapes are prone to episodic geomorphic adjustments related to long-term sediment accumulation and occasional reworking by flash floods. However, few studies document the rates, patterns and processes leading to channel breakdown, particularly avulsion and floodout. These intrinsically driven adjustments may pose a risk to water resource availability through redistribution of water and sediment on the floodplain. To address this knowledge gap, channel change associated with avulsion and flooding in the last ∼30 years were quantified for Little Faulkenhagen Creek, a dryland river of the Murray-Darling Basin, Australia. The semi-arid, low gradient catchment promotes transmission losses, which with loss of valley confinement leads to channel adjustment and contraction to occur where water, sediment and energy are spread across the floodplain. Pronounced downstream changes occur in channel capacity (i.e., reductions in bankfull width, depth and area) and hydrology (i.e., reductions in bankfull discharge and stream power) as the river enters the floodout zone. Vegetation roughness increases in and around the floodout, where shrubs and river redgum trees line and encroach upon the channel. Mapping and analysis of hydro-geomorphological parameters using cross sections from 1991 to 2021 reveal recent changes in channel capacity, discharge and stream power. The upper reach has experienced substantial channel widening from erosion, while the lower reach leading into the floodout has contracted because of sediment deposition within the channel. Unchannelised floodwaters are deflected around a raised alluvial ridge and over the floodout eventually dropping into several migrating upstream knickpoints (head-cutting gullies) with retreat rates up to 32 m a−1. Therefore, both the existing channel slope (i.e., decline) and potential avulsion courses (i.e., increase) are affected by reduced energy and enhanced sedimentation. The net result of upstream in-channel sedimentation and aggradation on the floodout and downstream incision and gullying is that the floodout has migrated ∼1 km upstream and the river is at risk of a secondary avulsion as gullies retreat towards the main channel, potentially circumventing the floodout within ∼11 years. An erosion cell conceptual model is suitable for this system, where sediment from the local upstream production (erosion) zone is transferred downstream to a sink (deposition) zone. Erosion and deposition will fluctuate over the long-term, leading to periods with dominantly incisional or aggradational processes. Cycles of erosion and deposition will vary over time and external forces may influence a wholesale shift to a new fluvial state. Understanding the context and patterns of recent dryland river adjustments, as well as the dominant processes and their variability, can assist water and land management.

A novel subsurface sediment plate method for quantifying sediment accumulation and erosion in seagrass meadows

Sediment dynamics in seagrass meadows are key determinants of carbon sequestration and storage, surface elevation, and resilience and recovery from disturbance. However, current methods for measuring sediment accumulation are limited. For example, 210Pb dating, the most popular tool for quantifying sediment accretion rates over decadal timescales, relies on assumptions often at odds with seagrass meadows. Here, we have developed a novel subsurface sediment plate method to detect changes in sediment accumulation and erosion in real time that: 1) is affordable and simple to implement, 2) can quantify short-term (weeks to months) sediment dynamics of accumulation and erosion, 3) is non-destructive and minimizes impacts to surface-level processes, and 4) can quantify long-term (years) net sediment accumulation rates. We deployed subsurface sediment plates at two sites within a 20 km2 seagrass meadow in the Virginia Coast Reserve Long-Term Ecological Research site, USA. Here, we discuss spatial and temporal trends in sediment dynamics over a 25-month period, the sediment accretion rates estimated using the subsurface sediment plate method compared to previous estimates based on 210Pb dating, the precision of the method, and our recommendations for implementing the method for measuring surface sediment dynamics in other seagrass settings. We recommend the application of this method for quantifying short- and long-term changes in seagrass surface sediments across various spatial scales to improve our understanding of disturbance, recovery, restoration, carbon cycling, sediment budgets, and the response of seagrasses to rising sea levels.

Fine-sediment Supply Can Control Fluvial Deposit Architecture: An Example From the Blackhawk Formation-Castlegate Sandstone Transition, Upper Cretaceous, Utah, USA

The arrangement of channel and floodplain deposits in alluvial basins reflects the balance of subsidence, sediment supply, and channel avulsion behavior during accumulation. Approaches for reconstructing tectonic and climatic histories from alluvial architecture generally assume that floodplain preservation is primarily a function of channel mobility relative to long-term sediment-accumulation rate; however, the amount of mud supplied to a river network can significantly impact the baseline accumulation of fine-grained deposits in alluvial basins. Here we evaluate preserved fine-sediment volume fractions at the bedform, reach, and outcrop scale across the transition from the mudstone-dominated Blackhawk Formation to the sandstone-dominated Lower Castlegate Sandstone (Upper Cretaceous, Utah, USA). Results show a nearly 50% decrease in mud abundance across the Blackhawk-Castlegate transition at a range of morphodynamic scales (mud percent in bed material: 28.4% to 14.1%, interbar fine deposits: 39.6% to 22.1%, and outcrop architecture: 58% to 16%). This decrease in fine-grained sediment coincides with an abrupt increase in quartz abundance from Blackhawk to Castlegate sands, suggesting that unroofing quartz-rich source rock caused significant regional changes in the alluvial deposits. This result shows that changes in sediment supply grain size are detectable from bed to landscape scales and can cause major changes in stratigraphic architecture. This method of comparing sand-to-mud ratios can be broadly applied in other fluvial successions and in source-to-sink transects to better reconstruct mud fluxes through ancient fluvial networks and to investigate how rivers respond to changes in fine-sediment availability.

Examining sediment infill dynamics at Naracoorte cave megafauna sites using multiple luminescence dating signals

Relatively little is known about the long-term sediment accumulation dynamics of Naracoorte Cave Complex (NCC) solution pipe cavities, and many of the megafauna-bearing infill deposits at this globally significant Australian Pleistocene fossil locality remain partially dated or lack any numerical age control. In this study, we assess the suitability of three different luminescence dating signals for improving existing chronologies at six Late and Middle Pleistocene NCC sites (n = 22 samples), and we undertake multi-site examinations of NCC sediment infill dynamics spanning the last 550 thousand years (ka). Modern analogue samples collected from above and beneath two active cave entrances confirm that single-grain OSL, single-grain TT-OSL and multi-grain pIR-IRSL signals can be reset down to insignificant residual levels (<10−1–100 Gy) when compared with the natural dose ranges of interest for most NCC palaeontological applications. Replicate luminescence dating comparisons performed at six NCC fossil sites (n = 15 samples) reveal consistent ages for twenty-eight out of thirty-one paired OSL–TT-OSL, OSL–pIR-IRSL and TT-OSL–pIR-IRSL datasets. Nineteen of the twenty Middle to Late Pleistocene samples analysed from the NCC sites produce homogeneous OSL, TT-OSL and pIR-IRSL De datasets suggesting that the NCC solution pipe deposits considered here are generally not affected by syn-depositional mixing complications that can take place within closed karst cavities (i.e., the remobilisation of unbleached grains from pre-existing cave floor sediments, and their subsequent translocation and incorporation into deposits within deeper parts of the cavity along with externally bleached, allochthonous grain populations). Detailed examination of solution pipe dynamics at Smoke Tortoise Cave (SMT) reveals a complex accumulation history focused on the marine isotope stage (MIS) 9 and MIS 7 interglacial complexes, as well as the MIS 8e interstadial. The SMT case study highlights that NCC solution pipes are not simply associated with short-lived opening and sediment accumulation events, but may involve multiple, discontinuous deposition episodes and reactivation events. An initial multi-site examination of all published NCC infill chronologies (n = 70) appears to suggest statistically significant, preferential solution pipe development during the relatively wet parts of interglacial or interstadial cycles. The non-uniform infill age distribution implies that NCC solution pipe dynamics may have exerted taphonomic biases on fossil accumulation, which should be taken into consideration when reconstructing long-term palaeoecological histories from NCC solution pipe cavities.

Distribution, morphology and composition of mesophotic ‘reefs’ on the Amazon Continental Margin

The geomorphology of the Amazon Continental Margin (ACM) is highly heterogeneous and includes a variety of reef-like formations found in deep-water along the shelf-slope transition. The ACM has been divided into three Sectors (Northern, Central and Southern) according to the distribution of the carbonate producers and the ‘reefs’, influenced by the Amazon River plume. Here, we characterize these structures that were sampled with a manned submersible and multibeam surveys to depths of up to 230 m, exploring their relationship with the Amazon river plume and changing sea level following the Last Glacial Maximum (LGM). The shelf-slope transition and a deeper shelf break in the Northern Sector of the Brazilian portion of the ACM carry a series of prominent high-relief structures (HRS) that experience a strong fluvial influence. The tops of these features are between 110 and 165 m depths and seem to have originated during lowstands, through erosion of Pleistocene sandstones. Siliciclastic and carbonate deposits accumulated on the tops of these features in shallow waters during the LGM and early deglaciation and were gradually submerged by rising sea level and subsidence. A thin layer of encrusting organisms, coralline algae, sponges, bryozoans and serpulids presently colonizes most of these surfaces at the sites sampled and contributes to the aggregation of thin fine-grained siliciclastic deposits. The Central Sector, off the river mouth, is associated with long-term sediment accumulation and lacks a shelf break. Nevertheless, living benthic communities occur on rock outcrops. The Southern Sector is less influenced by the river plume and includes a shallower shelf-break and a prominent canyon. The surveyed outcrops of Pleistocene carbonate and siliciclastic rocks at 180 m carry thin covers of a benthic community dominated by sponges and coralline algae, which is responsible for the accumulation of a thin (up to a few centimeters) deposit of carbonate bioclasts, quartz sand and mud on the rocky substrate. Thus, the ‘reefs’ of the outer shelf and mesophotic Amazon margin are typically eroded older rocks, colonized by encrusting organisms during the LGM and deglaciation forming a carbonate veneer, which now support a mesophotic community.

Quantifying water storage within the north of Lake Naivasha using sonar remote sensing and Landsat satellite data

Endorheic freshwater lakes can be vital water resources for sustaining large populations. However, their land-locked nature can lead to overexploitation and long-term sediment accumulation, reducing water storage and quality. Lake Naivasha supports a rapidly expanding population and agricultural industry. Therefore, maintaining good water storage and quality within this endorheic lake is crucial for the Kenyan economy and population. The lake has a long history of level fluctuations and the region is considered to be suffering from a chronic imbalance between water supply and demand.This study quantifies the sediment deposition rate and its impact on Lake Naivasha's water levels and volume, using inexpensive remote sensing techniques that could be easily replicated for future monitoring.Evidence of sedimentation in the northern area averaging 23 mm yr−1 was identified, which is likely annually displacing between 40.2 – 576 × 103 m³ of water. The volume displaced each year is equivalent to the water required to sustain between 40 – 1152 people. These results imply that current abstraction management, based purely upon lake level readings that govern a ‘traffic lights’ system, are detrimental to the long-term survival of the lake. The results also imply that lake health is decreasing. We recommend that future monitoring of this water resource and all endorheic lakes consider measurements of available water volume in combination with lake level data using the remote sensing methods we describe.

Comparative Analysis of Different Mobile LiDAR Mapping Systems for Ditch Line Characterization

Maintenance of roadside ditches is important to avoid localized flooding and premature failure of pavements. Scheduling effective preventative maintenance requires a reasonably detailed mapping of the ditch profile to identify areas in need of excavation to remove long-term sediment accumulation. This study utilizes high-resolution, high-quality point clouds collected by mobile LiDAR mapping systems (MLMS) for mapping roadside ditches and performing hydrological analyses. The performance of alternative MLMS units, including an unmanned aerial vehicle, an unmanned ground vehicle, a portable backpack system along with its vehicle-mounted version, a medium-grade wheel-based system, and a high-grade wheel-based system, is evaluated. Point clouds from all the MLMS units are in agreement within the ±3 cm range for solid surfaces and ±7 cm range for vegetated areas along the vertical direction. The portable backpack system that could be carried by a surveyor or mounted on a vehicle is found to be the most cost-effective method for mapping roadside ditches, followed by the medium-grade wheel-based system. Furthermore, a framework for ditch line characterization is proposed and tested using datasets acquired by the medium-grade wheel-based and vehicle-mounted portable systems over a state highway. An existing ground-filtering approach—cloth simulation—is modified to handle variations in point density of mobile LiDAR data. Hydrological analyses, including flow direction and flow accumulation, are applied to extract the drainage network from the digital terrain model (DTM). Cross-sectional/longitudinal profiles of the ditch are automatically extracted from the LiDAR data and visualized in 3D point clouds and 2D images. The slope derived from the LiDAR data turned out to be very close to the highway cross slope design standards of 2% on driving lanes, 4% on shoulders, and a 6-by-1 slope for ditch lines.

Long-Term Sediment, Carbon, and Nitrogen Accumulation Rates in Coastal Wetlands Impacted by Sea Level Rise

Coastal wetlands provide important ecosystem services, such as carbon and nitrogen sequestration and flood protection, but are vulnerable to increasing sea levels. We examined the vulnerability of coastal wetlands to rising sea levels using radioisotopes (210Pb and 137Cs) to estimate long-term (~100 years) soil accretion rates. We also estimated carbon and nitrogen accumulation rates. We collected soil cores from four transects along the Albemarle-Pamlico Peninsula of North Carolina, an area experiencing high rates of sea level rise. Transects included four wetland types: forested, transitional between forest and marsh, brackish marsh, and estuarine open water. Of the 16 sites we sampled, we were able to estimate accretion rates for 14 using 210Pb and four cores using 137Cs. Only one site had a vertical accretion rate higher than the local rate of sea level rise (0.45 cm year−1): a transitional wetland site with fine sediment sourced from a nearby canal (0.88 cm year−1). Mean vertical accretion across all sites was 0.20 cm year−1. Our results show that most wetlands in the area are vulnerable to future inundation due to current rates of sea level rise. Extrapolating from our measurement to the entire peninsula suggests that if wetlands have room to migrate and transition from forest to marsh, the potential for sea level rise–induced loss of C and N accumulation will be 2–20%, compared to 35–88% when not accounting for wetland transitions, illustrating the importance of allowing space for wetlands to migrate in these changing landscapes.

Geomorphometric Seabed Classification and Potential Megahabitat Distribution in the Amazon Continental Margin

The geomorphometry of the of the Amazon Continental Margin was analyzed using the Benthic Terrain Model, a spatial analysis technique that defines physical habitat classes based on seafloor relief heterogeneities. A compilation of available bathymetric and novel multibeam data was used to define the megahabitats, with emphasis on shelf-slope transitions and shelf-edge reefs. The analyses revealed a complex mosaic of benthic megahabitats associated to short and long-term geologic and sedimentary evolution of the margin. The continuous terrigenous sediment input is associated to a smooth muddy deposit along the inner and mid shelf (Regular Continental Shelf megahabitat). The portions of the shelf that are less influenced by riverine sediment accumulation are rougher and characterized by either sand (Irregular Sand Continental Shelf megahabitat) or carbonate-dominated bottom (Irregular Reef Continental Shelf megahabitat). The most notable difference in terms of morphometric analysis and megahabitats can be observed along the outer shelf and shelf-break. The Shelf-Slope Transition megahabitat is marked by ridges in the shelf break and by a more acute depth gradient that forms a distinct outer shelf edge. Three different alongshore sectors were explored in order describe the heterogeneous megahabitat mosaic in terms of slope and bottom morphology. The western-most sector (S3) is remarkable due to an indistinct separation between ridges and the outer shelf edge, as well as for presenting reefs with up to 20 m high, between 110 and 210 m water depths. The central sector (S2) presents no shelf-break and lacks ridges, a feature that is associated with the long-term sediment accumulation (Amazon Fan). The southern-most sector (S1) does not present an outer shelf edge, only ridges, and a number of shelf-incised channels, comprising a sediment bypass across the shelf, and carbonate sedimentation. The continental slope is divided into a Featured Slope megahabitat with numerous canyons and ravines, and areas that lack such features, including a Shallow Gentle Slope megahabitat, a Steep Slope megahabitat and a Deep Gentle Slope megahabitat. Our results confirm the usefulness of geomorphometric analyses to define benthic megahabitats and can be used as a starting point in a much-needed marine spatial planning process for the area.

Nature of the sedimentary rock record and its implications for Earth system evolution.

The sedimentary rock reservoir both records and influences changes in Earth's surface environment. Geoscientists extract data from the rock record to constrain long-term environmental, climatic and biological evolution, with the understanding that geological processes of erosion and rock destruction may have overprinted some aspects of their results. It has also long been recognized that changes in the mass and chemical composition of buried sediments, operating in conjunction with biologically catalyzed reactions, exert a first-order control on Earth surface conditions on geologic timescales. Thus, the construction and destruction of the rock record has the potential to influence both how Earth and life history are sampled, and drive long-term trends in surface conditions that otherwise are difficult to affect. However, directly testing what the dominant process signal in the sedimentary record is - rock construction or destruction - has rarely been undertaken, primarily due to the difficulty of assembling data on the mass and age of rocks in Earth's crust. Here, we present results on the chronological age and general properties of rocks and sediments in the Macrostrat geospatial database (https://macrostrat.org). Empirical patterns in surviving rock quantity as a function of age are indicative of both continual cycling (gross sedimentation) and long-term sediment accumulation (net sedimentation). Temporal variation in the net sedimentary reservoir was driven by major changes in the ability of continental crust to accommodate sediments. The implied history of episodic growth of sediment mass on continental crust has many attendant implications for the drivers of long-term biogeochemical evolution of Earth and life.

Differences in long-term vs short-term carbon and nitrogen sequestration in a coastal river delta wetland: Implications for global budgets

Coastal wetlands can serve as a considerable sink for carbon (C) gases. However, the capacity for wetlands to serve as more permanent C and N sinks over the long term is less clear given the time dependence of sediment deposition, organic matter decomposition, and anthropogenic land use change. In this study, we compare the short-term (decadal scale) and long-term (millennial scale) C and N accumulation rates estimated using 137Cs and radiocarbon dating of vibracores collected from a freshwater coastal wetland in the Louisiana Mississippi River deltaic plain (Atchafalaya River delta). The mean short-term (60 yrs) sediment accumulation rate was 1.4 cm/yr while the mean rate of long-term (100–1000 yrs) sediment accumulation was an order of magnitude lower at 0.12 cm/yr. Annual rates of C and N accumulated over the past several thousand years were approximately 10% of that over the past 60 years after correcting for bulk density. These results are similar to other coastal wetlands and suggest that time scale must be considered in determining the relative permanence of C and N storage in coastal wetland soils. This difference is especially important for assessing the role of C cycling in relation to global change models and N cycling related to water quality in accurately quantifying the role of coastal deltaic fresh water wetlands in regulating these biogeochemical cycles.

Pliocene and Pleistocene chronostratigraphy of continental sediments underlying the Altiplano at La Paz, Bolivia

Continental sediments underlying the Altiplano plateau provide insight into the late Cenozoic evolution of the Central Andes. We characterize the magnetostratigraphy and lithostratigraphy of the upper part of this fill sequence along a transect extending southwestward from the Cordillera Real at La Paz, Bolivia, where it is best exposed. Multiple polarity reversals and the locally extensive, 2.74-Ma Chijini Tuff enable correlation between our six sections and three previously reported sections. The tuff ties the composite polarity sequence to the geomagnetic polarity time scale, demonstrating that the stratigraphic record extends from the latest Gilbert Chron (ca. 3.8 Ma) to the late Olduvai subchron (ca. 1.8 Ma), or possibly Jaramillo subchron (ca. 1.0 Ma). The sequence provides Earth's longest known record of low-latitude glaciation and the only record of Pliocene tropical glaciation. It includes evidence for 16 late Pliocene and Early Pleistocene glaciations, separated by interglacials of sufficient length (>103-104 a) to produce mature soil profiles. Successively larger ice caps formed directly before, during, and after the globally warm mid-Piacenzian (3.265–3.025 Ma), and throughout Plio-Pleistocene climate deterioration. The late Pliocene glacial units predate the onset of widespread Northern Hemisphere continental glaciation and in most cases unambiguously correspond to specific cool peaks of the astronomically tuned, benthic oxygen isotope (δ18O) record, including marine isotope stages MG2, M2, KM2, and G10. The glacial events broadly coincide with those nearer both poles, suggesting inter-hemispheric climate linkages. The early formation and subsequent expansion of ice caps beyond glacier margins of the Last Glacial Maximum suggest that the Cordillera Real likely attained its modern height before ca. 3.4 Ma. The number and timing of glaciations, and long-term sediment accumulation and incision rates suggest that the local Altiplano surface formed by ca. 1.8 Ma, after which headwaters of the Amazon River system breached the Cordillera Real and began to incise the fill sequence. The sequence spans at least 2 Ma before and during the first main pulses of the Great American Biotic Interchange, providing an important record of faunal migration. The arrival of savanna-adapted North American mammals in the Central Andes during this period points to the influence of glaciation on faunal migration in the southern tropics, similar to influences proposed in North and Central America.

Chemostratigraphic age model for the Tornillo Group: A possible link between fluvial stratigraphy and climate

The expression of global climate trends (106 to 107years) and events (105years) in terrestrial sedimentary sections can only be assessed from long, continuous continental records. Such records are rare due to the paucity of well-dated terrestrial deposits covering millions of years. This study uses isotope chemostratigraphy to develop an age model for fluvial sedimentary rocks of the Tornillo Group (Tornillo Basin, TX) and to decipher the potential expression of global climate variations in this sub-tropical paleo-fluvial system. The carbon isotope ratio (δ13C) of Tornillo Group pedogenic carbonate correlates well with δ13C variations from benthic foraminifera and suggests deposition between ~69Ma and ~52Ma for the studied stratigraphic interval. Higher sediment accumulation rates occurred in the greenhouse period of the early Eocene than during the cooler middle Paleocene. Carbon isotope excursions associated with Eocene hyperthermals were not identified, but their predicted stratigraphic positions coincide with thick sand bodies. We interpret the long-term sediment accumulation rates and rapid shifts in facies distribution as reflecting change in sediment supply in the basin, possibly driven by fluctuations in precipitation seasonality and intensity driven associated with global temperature changes. Carbon and oxygen isotope ratios from pedogenic carbonates display a strong correlation throughout the section, interpreted as an expression of coupling between the hydrological cycle and soil processes. Those observations suggest that global climate variations, along with tectonic and eustatic sea-level changes, may be an important control of stratigraphic variations of the Tornillo Group at multiple timescales.

Contemporary Deposition and Long-Term Accumulation of Sediment and Nutrients by Tidal Freshwater Forested Wetlands Impacted by Sea Level Rise

Contemporary deposition (artificial marker horizon, 3.5 years) and long-term accumulation rates (210Pb profiles, ~150 years) of sediment and associated carbon (C), nitrogen (N), and phosphorus (P) were measured in wetlands along the tidal Savannah and Waccamaw rivers in the southeastern USA. Four sites along each river spanned an upstream-to-downstream salinification gradient, from upriver tidal freshwater forested wetland (TFFW), through moderately and highly salt-impacted forested wetlands, to oligohaline marsh downriver. Contemporary deposition rates (sediment, C, N, and P) were greatest in oligohaline marsh and lowest in TFFW along both rivers. Greater rates of deposition in oligohaline and salt-stressed forested wetlands were associated with a shift to greater clay and metal content that is likely associated with a change from low availability of watershed-derived sediment to TFFW and to greater availability of a coastal sediment source to oligohaline wetlands. Long-term accumulation rates along the Waccamaw River had the opposite spatial pattern compared to contemporary deposition, with greater rates in TFFW that declined to oligohaline marsh. Long-term sediment and elemental mass accumulation rates also were 3–9× lower than contemporary deposition rates. In comparison to other studies, sediment and associated nutrient accumulation in TFFW are lower than downriver/estuarine freshwater, oligohaline, and salt marshes, suggesting a reduced capacity for surface sedimentation (short-term) as well as shallow soil processes (long-term sedimentation) to offset sea level rise in TFFW. Nonetheless, their potentially large spatial extent suggests that TFFW have a large impact on the transport and fate of sediment and nutrients in tidal rivers and estuaries.

What Role do Hurricanes Play in Sediment Delivery to Subsiding River Deltas?

The Mississippi River Delta (MRD) has undergone tremendous land loss over the past century due to natural and anthropogenic influences, a fate shared by many river deltas globally. A globally unprecedented effort to restore and sustain the remaining subaerial portions of the delta is now underway, an endeavor that is expected to cost $50–100B over the next 50 yr. Success of this effort requires a thorough understanding of natural and anthropogenic controls on sediment supply and delta geomorphology. In the MRD, hurricanes have been paradoxically identified as both substantial agents of widespread land loss, and vertical marsh sediment accretion. We present the first multi-decadal chronostratigraphic assessment of sediment supply for a major coastal basin of the MRD that assesses both fluvial and hurricane-induced contributions to sediment accumulation in deltaic wetlands. Our findings indicate that over multidecadal timescales, hurricane-induced sediment delivery may be an important contributor for deltaic wetland vertical accretion, but the contribution from hurricanes to long-term sediment accumulation is substantially less than sediment delivery supplied by existing and planned river-sediment diversions at present-day river-sediment loads.

Late Holocene mangrove development and onset of sedimentation in the Yax Chen cave system (Ox Bel Ha) Yucatan, Mexico: Implications for using cave sediments as a sea-level indicator

Abstract This study examines the relationship between the flooding of cenotes and formation of coastal mangrove with Holocene sea-level rise and the onset of aquatic sedimentation in Yax Chen, a cave system in Quintana Roo on Mexico's Yucatan Peninsula. Sediment depth measurements (n = 180) were collected along 2.7 km of an underwater cave passage and three cores were radiocarbon dated to examine both the extent and timing of sedimentation in the cave. Basal radiocarbon ages (~ 4 Ka) for aquatic sediments in the cave show that Holocene sea-level rise flooded cenotes, creating sunlit open water conditions with associated mangroves on the upper karst surface. These conditions initiated abundant and widespread sedimentation in the cave. Cenote surface area controlled the long-term sediment accumulation in the cave passages through primary productivity in the sunlit open water areas of the cenotes. This primary productivity was enhanced with mangrove formation, which causes funneling of precipitation and nutrient-rich waters into the cenotes from the mangroves. Accumulation histories from the radiocarbon-dated sediment cores (n = 3) were compared with accumulation histories in previously published studies including Actun Ha, Mexico and Green Bay Cave (GBC), Bermuda.

A preliminary OSL chronology for coastal dunes on Moreton island, Queensland, Australia – Marginal deposits of A large-scale quaternary shelf sediment system

Moreton Island and several other large siliceous sand dune islands and mainland barrier deposits in SE Queensland represent the distal, onshore component of an extensive Quaternary continental shelf sediment system. This sediment has been transported up to 1000km along the coast and shelf of SE Australia over multiple glacioeustatic sea-level cycles. Stratigraphic relationships and a preliminary Optically Stimulated Luminance (OSL) chronology for Moreton Island indicate a middle Pleistocene age for the large majority of the deposit. Dune units exposed in the centre of the island and on the east coast have OSL ages that indicate deposition occurred between approximately 540ka and 350kaBP, and at around 96±10kaBP. Much of the southern half of the island has a veneer of much younger sediment, with OSL ages of 0.90±0.11ka, 1.28±0.16ka, 5.75±0.53ka and <0.45kaBP. The younger deposits were partially derived from the reworking of the upper leached zone of the much older dunes. A large parabolic dune at the northern end of the island, OSL age of 9.90±1.0kaBP, and palaeosol exposures that extend below present sea level suggest the Pleistocene dunes were sourced from shorelines positioned several to tens of metres lower than, and up to few kilometres seaward of the present shoreline. Given the lower gradient of the inner shelf a few km seaward of the island, it seems likely that periods of intermediate sea level (e.g. ~20m below present) produced strongly positive onshore sediment budgets and the mobilisation of dunes inland to form much of what now comprises Moreton Island. The new OSL ages and comprehensive OSL chronology for the Cooloola deposit, 100km north of Moreton Island, indicate that the bulk of the coastal dune deposits in SE Queensland were emplaced between approximately 540kaBP and prior to the Last Interglacial. This chronostratigraphic information improves our fundamental understanding of long-term sediment transport and accumulation on large-scale continental shelf sediment systems.

Sediment accumulation, stratigraphic order, and the extent of time-averaging in lagoonal sediments: a comparison of 210Pb and 14C/amino acid racemization chronologies

Carbon-14 calibrated amino acid racemization (14C/AAR) data and lead-210 (210Pb) data are used to examine sediment accumulation rates, stratigraphic order, and the extent of time-averaging in sediments collected from the One Tree Reef lagoon (southern Great Barrier Reef, Australia). The top meter of lagoonal sediment preserves a stratigraphically ordered deposit spanning the last 600 yrs. Despite different assumptions, the 210Pb and 14C/AAR chronologies are remarkably similar indicating consistency in sedimentary processes across sediment grain sizes spanning more than three orders of magnitude (0.1–10 mm). Estimates of long-term sediment accumulation rates range from 2.2 to 1.2 mm yr−1. Molluscan time-averaging in the taphonomically active zone is 19 yrs, whereas below the depth of final burial (~15 cm), it is ~110 yrs/5 cm layer. While not a high-resolution paleontological record, this reef lagoon sediment is suitable for paleoecological studies spanning the period of Western colonization and development. This sedimentary deposit, and others like it, should be useful, albeit not ideal, for quantifying anthropogenic impacts on coral reef systems.