Changes In Sediment Accumulation Rates Research Articles

A century of human interventions on sediment flux variations in the Danube-Black Sea transition zone

Many deltas around the world have recorded a decreasing sediment input, mostly due to retention in dams constructed on the river or in the river basin. The Danube River has recorded a significant decrease of its sediment supply to the Danube Delta and the NW Black Sea. This study uses 210Pb and 137Cs dating, to investigate the effects of the decreasing sediment flux in lakes, lagoons, delta front and prodelta area. Both the effects of the Iron Gate I and II dams and other local factors are discussed. These results define the period of 1960-1990 as the ‘major anthropic interventions period’ in the Danube Delta. Results show a decrease in siliciclastic flux, especially in lakes, the Sahalin lagoon and the prodelta area and a general increase in the Musura lagoon and the delta front area. Sand content is also shown to decrease in most areas and is replaced by silt. The changes in sediment accumulation rates depend mostly on the hydrological connectivity of the area and the local hydrotechnical works. Overall, the local anthropic interventions in the delta affect sediment flux in the subaerial delta and the delta front, while the prodelta is affected by the overall decrease caused by interventions in the river basin. This study can contribute to improving management strategies in the area and to a better understanding of future research needs in the Danube Delta-Black Sea system.

Using sediment accumulation rates in floodplain paleochannel lakes to reconstruct climate-flood relationships on the lower Ohio River

Late Holocene flood frequencies on the lower Ohio River were investigated using 14C-based sedimentation rates from three floodplain lakes located in Illinois (Avery Lake), Kentucky (Grassy Pond), and Indiana (Goose Pond). Changes in sediment accumulation rates were attributed to variability in the delivery of overbank sediment to each site as controlled by the frequency of Ohio River flooding. Sedimentation rates reached their lowest values in all three lakes between 400 and 1230 CE, indicating a regional reduction in flood frequencies on the lower Ohio River during a period that included the Medieval Climate Anomaly (MCA; ca. 950–1250 CE). Sedimentation rates increased after ca. 1230 CE and remained moderately high through the Little Ice Age (LIA; 1350–1820 CE) until the onset of extensive land clearance during the early 1800s CE. After 1820 CE, sedimentation rates increased further and were higher than any other time during the late Holocene. A comparison of regional paleoclimatic proxies with the above floodplain sedimentation records shows that Ohio River flooding during the late Holocene was responsive to mean-state changes in atmospheric circulation. During the MCA, when clockwise mean-state atmospheric circulation advected southerly moisture from the Gulf of Mexico into the Ohio River Valley primarily in the form of convective rainstorms, flooding on the Ohio River was least frequent. During the LIA, meridional mean-state atmospheric circulation increased the proportion of midcontinental moisture that was sourced from the northern Pacific and Arctic and delivered as snowfall, hence increasing flooding on the Ohio River. We attribute the increase in Ohio River flooding during the LIA to an increase in snowpack volume across the Ohio River Valley and the watershed-scale integration of runoff during spring snowmelt. Following Euro-American land clearance in the early 1800s, flood frequencies decoupled from this relationship and the lower Ohio River became susceptible to frequent flooding, despite a return to southerly and clockwise synoptic atmospheric conditions. These modern climate-flood dynamics are fundamentally different than those of the paleo-record and suggest that land-use changes – such as deforestation, tile draining, and landscape conversion to intensive row crop agriculture – have fundamentally altered the modern Midwestern hydrologic cycle.

Evaluating the Evolution of Soil Erosion under Catchment Farmland Abandonment Using Lakeshore Sediment

Discriminating the potential sources contributing to lacustrine sediment is helpful for decision-making for catchment soils and lake management strategies within lake-catchment systems. Using a sediment fingerprinting approach from the multivariate mixing model, the spatiotemporal sources of geology and land use were identified in a small agricultural catchment in southwest China. Results showed that sediment accumulation rates (SARs) were estimated to range from 0.002 to 0.065 g cm−2 a−1 (mean 0.015 ± 0.016 g cm−2 a−1), which has a positive correlation with instrumental Indian Summer Monsoon (ISM) precipitation. Time-integrated sources were divided into four zones in combination with the changes in SARs, which were qualitatively and quantitively interpreted by particle size, and precipitation, and historical land use polies over the past ~160 years. Spatially, Quaternary granite (QG) in geology and channel bank (CB) in land use were the dominant contributors to the lakeshore sediment, respectively. Two relatively higher contributions of abandoned land (AL) to lakeshore sediment were found during the periods of 1930s–1950s and post-1990s, which originated from the dual impacts of topographical factors of slope gradient and elevation, and socioeconomic factors of the gap of farmer’s expenditure to income. The results illustrate that restricting the farmland to be abandoned would be useful for reducing the soil erosion within the lake-catchment system.

Holocene evolution of the wave-dominated embayed Moruya coastline, southeastern Australia: Sediment sources, transport rates and alongshore interconnectivity

Sediment budgets on wave-dominated coastlines are important in determining shoreline behaviour and are primarily inherited from geological-scale coastal evolutionary history. Sediment compartments provide a framework to conceptualise and investigate sediment budgets over a range of time and space scales. This study aims to assess the sediment budget for a secondary coastal compartment on the New South Wales (NSW) south coast ∼26 km in length and containing five adjacent but discrete Holocene coastal bay barriers: Barlings Beach, Broulee Beach, Bengello Beach, Moruya Heads Beach and Pedro Beach. Building on earlier morphostratigraphic studies, a new series of Optically Stimulated Luminescence (OSL) ages are presented for foredune ridge sequences at previously un-dated sites. Additional Ground Penetrating Radar (GPR) transects complement earlier stratigraphic datasets, and seamless topographic and bathymetric LiDAR datasets provide insight into subaerial coastal deposits and inner shelf morphology in this region. The results demonstrate that barriers within the compartment have two different sediment sources. The barriers are dominated by shoreface supply of quartz-rich sand transported onshore as shoreface morphology evolved towards equilibrium. Skeletal carbonate sand became an important component of the sediment budget for the northern Barlings and Broulee beaches after ∼3000 years ago. Shoreline progradation at Bengello Beach has been steady throughout the mid-to late- Holocene. Bengello contains the largest volume of Holocene sand and accreted at an average rate of 3.1 m3/m/yr (for the current shoreline length). The other barrier systems have experienced changes in sediment accumulation rate as their shorelines prograded seaward resulting in changes to their alongshore interconnectivity. Rapid filling of the Pedro Beach embayment by ∼4000 years ago initiated headland bypassing and northward sand transport to Moruya Heads Beach which only then commenced progradation. In contrast, as Broulee and Bengello Beaches prograded they converged in the lee of Broulee Island forming a tombolo which led to division of the former continuous shoreline into two. A marked increase in skeletal carbonate content at Broulee occurred after this separation attesting to the restriction of the southern quartz-sand source replaced by local carbonate production. This study emphasises the importance of understanding the long-term temporal variability in sediment budget and embayment interconnectivity in determining shoreline response to changing boundary conditions such as sea level and wave climate as well as contemporary anthropogenic influences.

Deciphering relationships between the Nicobar and Bengal submarine fans, Indian Ocean

The Nicobar Fan and Bengal fans can be considered as the eastern and western parts, respectively, of the largest submarine-fan system in the world. This study presents the integrated results of petrographic and provenance studies from the Nicobar Fan and evaluates these in the context of controls on sedimentation. Both fans were predominantly supplied by Himalaya-derived material from the main tectono-stratigraphic sequences as well as the Gangdese arc. A lack of volcanic material in the Nicobar Fan rules out sources from the Sumatra magmatic arc. Overall, the petrographic data shows a progressive decrease in sedimentary detritus and corresponding increase of higher-grade metamorphic detritus up-section. Changes in sediment provenance and exhumation rates in the Himalaya are seen to track changes in sediment accumulation rates. High sediment accumulation rates in the Bengal Fan occurred at ∼13.5–8.3 Ma, and in the Nicobar Fan from ∼9.5–5 Ma. Both fans show peak accumulation rates at 9.5–8.3 Ma (but with the Nicobar Fan being about twice as high), and both record a sharp drop from ∼5.5–5.2 Ma, that coincided with a change in river drainage associated with the Brahmaputra River diverting west of the uplifting Shillong Plateau. At ∼5 Ma, the Nicobar Fan was supplied by an eastern drainage route that finally closed at ∼2 Ma, when sediment accumulation rates in the Nicobar Fan significantly decreased. Sediment provenance record these changes in routing whereby Bengal Fan deposits include granitoid sources from the Namche Barwa massif in the eastern syntaxis that are not seen in the Nicobar Fan, likely due to a more localised eastern drainage that included material from the Indo-Burman wedge. Prior to ∼3 Ma, source exhumation rates were rapid and constant and the short lag-time rules out significant intermediate storage and mixing. In terms of climate versus tectonic controls, tectonically driven changes in the river network have had most influence on fan sedimentation.

Re-evaluating <sup>14</sup>C dating accuracy in deep-sea sediment archives

Abstract. The current geochronological state of the art for applying the radiocarbon (14C) method to deep-sea sediment archives lacks key information on sediment bioturbation. Here, we apply a sediment accumulation model that simulates the sedimentation and bioturbation of millions of foraminifera, whereby realistic 14C activities (i.e. from a 14C calibration curve) are assigned to each single foraminifera based on its simulation time step. We find that the normal distribution of 14C age typically used to represent discrete-depth sediment intervals (based on the reported laboratory 14C age and measurement error) is unlikely to be a faithful reflection of the actual 14C age distribution for a specific depth interval. We also find that this deviation from the actual 14C age distribution is greatly amplified during the calibration process. Specifically, we find a systematic underestimation of total geochronological error in many cases (by up to thousands of years), as well as the generation of age–depth artefacts in downcore calibrated median age. Even in the case of “perfect” simulated sediment archive scenarios, whereby sediment accumulation rate (SAR), bioturbation depth, reservoir age and species abundance are all kept constant, the 14C measurement and calibration processes generate temporally dynamic median age–depth artefacts on the order of hundreds of years – whereby even high SAR scenarios (40 and 60 cm kyr−1) are susceptible. Such age–depth artefacts can be especially pronounced during periods corresponding to dynamic changes in the Earth's Δ14C history, when single foraminifera of varying 14C activity can be incorporated into single discrete-depth sediment intervals. For certain lower-SAR scenarios, we find that downcore discrete-depth true median age can systematically fall outside the calibrated age range predicted by the 14C measurement and calibration processes, thus leading to systematically inaccurate age estimations. In short, our findings suggest the possibility of 14C-derived age–depth artefacts in the literature. Furthermore, since such age–depth artefacts are likely to coincide with large-scale changes in global Δ14C, which themselves can coincide with large-scale changes in global climate (such as the last deglaciation), 14C-derived age–depth artefacts may have been previously incorrectly attributed to changes in SAR coinciding with global climate. Our study highlights the need for the development of improved deep-sea sediment 14C calibration techniques that include an a priori representation of bioturbation for multi-specimen samples.

SEAMUS (v1.20): a Δ<sup>14</sup>C-enabled, single-specimen sediment accumulation simulator

Abstract. The systematic bioturbation of single particles (such as foraminifera) within deep-sea sediment archives leads to the apparent smoothing of any temporal signal as recorded by the downcore, discrete-depth mean signal. This smoothing is the result of the systematic mixing of particles from a wide range of depositional ages into the same discrete-depth interval. Previous sediment models that simulate bioturbation have specifically produced an output in the form of a downcore, discrete-depth mean signal. However, palaeoceanographers analysing the distribution of single foraminifera specimens from sediment core intervals would be assisted by a model that specifically evaluates the effect of bioturbation upon single specimens. Taking advantage of advances in computer memory, the single-specimen SEdiment AccuMUlation Simulator (SEAMUS) was created for MATLAB and Octave, allowing for the simulation of large arrays of single specimens. This model allows researchers to analyse the post-bioturbation age heterogeneity of single specimens contained within discrete-depth sediment core intervals and how this heterogeneity is influenced by changes in sediment accumulation rate (SAR), bioturbation depth (BD) and species abundance. The simulation also assigns a realistic 14C activity to each specimen, by considering the dynamic Δ14C history of the Earth and temporal changes in reservoir age. This approach allows for the quantification of possible significant artefacts arising when 14C-dating multi-specimen samples with heterogeneous 14C activity. Users may also assign additional desired carrier signals to single specimens (stable isotopes, trace elements, temperature, etc.) and consider a second species with an independent abundance. Finally, the model can simulate a virtual palaeoceanographer by randomly picking whole specimens (whereby the user can set the percentage of older, “broken” specimens) of a prescribed sample size from discrete depths, after which virtual laboratory 14C dating and 14C calibration is carried out within the model. The SEAMUS bioturbation model can ultimately be combined with other models (proxy and ecological models) to produce a full climate-to-sediment model workflow, thus shedding light on the total uncertainty involved in palaeoclimate reconstructions based on sediment archives.

Chronostratigraphy of the Baringo-Tugen Hills-Barsemoi (HSPDP-BTB13-1A) core – 40Ar/39Ar dating, magnetostratigraphy, tephrostratigraphy, sequence stratigraphy and Bayesian age modeling

The Baringo-Tugen Hills-Barsemoi 2013 drillcore (BTB13), acquired as part of the Hominin Sites and Paleolakes Drilling Project, recovered 228 m of fluvio-lacustrine sedimentary rocks and tuffs spanning a ~3.29–2.56 Ma interval of the highly fossiliferous and hominin-bearing Chemeron Formation, Tugen Hills, Kenya. Here we present a Bayesian stratigraphic age model for the core employing chronostratigraphic control points derived from 40Ar/39Ar dating of tuffs from core and outcrop, 40Ar/39Ar age calibration of related outcrop diatomaceous units, and core magnetostratigraphy. The age model reveals three main intervals with distinct sediment accumulation rates: an early rapid phase from 3.2–2.9 Ma; a relatively slow phase from 2.9–2.7 Ma; and the highest rate of accumulation from 2.7–2.6 Ma. The intervals of rapid accumulation correspond to periods of high Earth orbital eccentricity, whereas the slow accumulation interval corresponds to low eccentricity at 2.9–2.7 Ma, suggesting that astronomically mediated climate processes may be responsible for the observed changes in sediment accumulation rate. Lacustrine transgression-regression events, as delineated using sequence stratigraphy, dominantly operate on precession scale, particularly within the high eccentricity periods. A set of erosively based fluvial conglomerates correspond to the 2.9–2.7 Ma interval, which could be related to either the depositional response to low eccentricity or to the development of unconformities due to local tectonic activity. Age calibration of core magnetic susceptibility and gamma density logs indicates a close temporal correspondence between a shift from high- to low-frequency signal variability at ~3 Ma, approximately coincident the end of the mid-Piacenzian Warm Period, and the beginning of the cooling of world climate leading to the initiation of Northern Hemispheric glaciation c. 2.7 Ma. BTB13 and the Baringo Basin records may thus provide evidence of a connection between high-latitude glaciation and equatorial terrestrial climate toward the end of the Pliocene.

WITHDRAWN: Chronostratigraphy of the Baringo-Tugen-Barsemoi (HSPDP-BTB13-1A) core – 40Ar/39Ar dating, magnetostratigraphy, tephrostratigraphy, sequence stratigraphy and Bayesian age modeling

The Baringo-Tugen-Barsemoi 2013 drillcore (BTB13), acquired as part of the Hominin Sites and Paleolakes Drilling Project, recovered 228 m of fluviolacustrine sedimentary rocks and tuffs spanning a ~3.29–2.56 Ma interval of the highly fossiliferous and hominin-bearing Chemeron Formation, Tugen Hills, Kenya. Here we present a Bayesian stratigraphic age model for the core employing chronostratigraphic control points derived from 40Ar/39Ar dating of tuffs from core and outcrop, 40Ar/39Ar age calibration of related outcrop diatomaceous units, and core magnetostratigraphy. The age model reveals three main intervals with distinct sediment accumulation rates: an early rapid phase from 3.2 to 2.9 Ma; a relatively slow phase from 2.9 to 2.7 Ma; and the highest rate of accumulation from 2.7 to 2.6 Ma. The intervals of rapid accumulation correspond to periods of high Earth orbital eccentricity, whereas the slow accumulation interval corresponds to low eccentricity at 2.9–2.7 Ma, suggesting that astronomically mediated climate processes may be responsible for the observed changes in sediment accumulation rate. Lacustrine transgression-regression events, as delineated using sequence stratigraphy, dominantly operate on precession scale, particularly within the high eccentricity periods. A set of erosively based fluvial conglomerates correspond to the 2.9–2.7 Ma interval, which could be related to either the depositional response to low eccentricity or to the development of unconformities due to local tectonic activity. Age calibration of core magnetic susceptibility and gamma density logs indicates a close temporal correspondence between a shift from high- to low-frequency signal variability at ~3 Ma, approximately coincident the end of the mid-Piacenzian Warm Period, and the beginning of the cooling of world climate leading to the initiation of Northern Hemispheric glaciation c. 2.7 Ma. BTB13 and the Baringo Basin records may thus provide evidence of a connection between high-latitude glaciation and equatorial terrestrial climate toward the end of the Pliocene.

A decline in benthic foraminifera following the deepwater horizon event in the northeastern Gulf of Mexico.

Sediment cores were collected from three sites (1000–1200 m water depth) in the northeastern Gulf of Mexico from December 2010 to June 2011 to assess changes in benthic foraminiferal density related to the Deepwater Horizon (DWH) event (April-July 2010, 1500 m water depth). Short-lived radioisotope geochronologies (210Pb, 234Th), organic geochemical assessments, and redox metal concentrations were determined to relate changes in sediment accumulation rate, contamination, and redox conditions with benthic foraminiferal density. Cores collected in December 2010 indicated a decline in density (80–93%). This decline was characterized by a decrease in benthic foraminiferal density and benthic foraminiferal accumulation rate (BFAR) in the surface 10 mm relative to the down-core mean in all benthic foraminifera, including the dominant genera (Bulimina spp., Uvigerina spp., and Cibicidoides spp.). Cores collected in February 2011 documented a site-specific response. There was evidence of a recovery in the benthic foraminiferal density and BFAR at the site closest to the wellhead (45 NM, NE). However, the site farther afield (60 NM, NE) recorded a continued decline in benthic foraminiferal density and BFAR down to near-zero values. This decline in benthic foraminiferal density occurred simultaneously with abrupt increases in sedimentary accumulation rates, polycyclic aromatic hydrocarbon (PAH) concentrations, and changes in redox conditions. Persistent reducing conditions (as many as 10 months after the event) in the surface of these core records were a possible cause of the decline. Another possible cause was the increase (2–3 times background) in PAH’s, which are known to cause benthic foraminifera mortality and inhibit reproduction. Records of benthic foraminiferal density coupled with short-lived radionuclide geochronology and organic geochemistry were effective in quantifying the benthic response and will continue to be a valuable tool in determining the long-term effects of the DWH event on a larger spatial scale.

Characterizing the Deep Sea Benthic Foraminifera Impact and Response to the Deepwater Horizon Blowout in the Northeastern Gulf of Mexico

Sediment cores were collected from the northeastern Gulf of Mexico to assess changes in deep-sea benthic foraminifera related to the Deepwater Horizon blowout, which occurred from April to July of 2010. Short-lived radioisotope geochronology (210Pb and 234Th) and organic geochemical toxicity assessments were also made to relate changes in sediment accumulation rate and sediment toxicity levels with benthic foraminifera. Cores collected in December 2010 indicate a community-wide event characterized by a decrease in concentration and benthic foraminifera mass accumulation rate (BFMAR) in the surface 10 mm relative to the down-core mean in all benthic foraminifera. Cores collected in February 2011 document a site-specific response and possible community recovery. In the site closer to the wellhead (45 NM, NE) there is evidence of a recovery in the benthic foraminifera community in both the concentrations and BFMAR. However, the site farther afield (60 NM, NE) records a continued depletion event characterized by a further decrease in benthic foraminifera concentrations and BFMAR down to zero values. Among the many questions to address, [(1) the mechanism for the decline (anoxia, hydrocarbon toxicity, etc.), (2) the trophic implications and (3) the spatial extent of the decline] perhaps the most important is the determination of recovery time for the system. Further analysis of benthic foraminifera abundance from cores collected after February 2011 (September 2011 and August 2012) will address the pressing issue of how long it will take for the benthic communities to recover from an event such as the Deepwater Horizon blowout. The records of benthic foraminifera abundance coupled with short-lived radionuclide geochronology and organic geochemical toxicity has shown to be effective in documenting and quantifying the benthic community response and will continue to be a valuable tool in determining the long-term effects of the Deepwater Horizon oil blowout on a larger spatial scale.

Increased sediment loads over coral reefs in Saint Lucia in relation to land use change in contributing watersheds

Increased sedimentation is widely acknowledged to be an important stressor for Caribbean coral reefs. However, for most locations we currently lack both accurate records of changes in sediment accumulation rates over reefs as well as a quantitative link between land-based sources of sediment and sediment delivery to coastal waters. This paper aims to address this gap in our quantitative understanding of these processes for two watersheds in the island of Saint Lucia in the West Indies. We used sediment cores collected near downstream coral reefs to examine changes in sediment composition and accumulation rate over the past several decades and relied upon a GIS-based sediment budget model to estimate recent sediment yields in the two focal watersheds. Analysis of sediment cores indicated that accumulation rates of terrigenous sediment, originating from the upstream watersheds, and calcareous sediment, likely arising from dead corals, have increased 2–3 fold over the last 3–4 decades. Model-estimated changes in sediment yields between 1995 and 2010 were associated with the expansion of the unpaved road network and were congruent with measured changes in terrigenous sediment accumulation rates near the reefs over the same period. The majority (83–95%) of sediment yield in the two watersheds was attributable to unpaved and degraded roads; in fact, just four or five road segments, representing <20% of the road network in each watershed, accounted for nearly half of the estimated sediment yield in 2010. Our results suggest that unpaved roads are major sediment sources in the two study watersheds and therefore merit closer attention when implementing erosion control measures intended to reduce sediment loading into reef-bearing coastal waters.

An estimate of post-depositional remanent magnetization lock-in depth in organic rich varved lake sediments

We studied the paleomagnetic properties of relatively organic rich, annually laminated (varved) sediments of Holocene age in Gyltigesjön, which is a lake in southern Sweden. An age–depth model was based on a regional lead pollution isochron and Bayesian modelling of radiocarbon ages of bulk sediments and terrestrial macrofossils, which included a radiocarbon wiggle-matched series of 873 varves that accumulated between 3000 and 2000 Cal a BP (Mellström et al., 2013). Mineral magnetic data and first order reversal curves suggest that the natural remanent magnetization is carried by stable single-domain grains of magnetite, probably of magnetosomal origin. Discrete samples taken from overlapping piston cores were used to produce smoothed paleomagnetic secular variation (inclination and declination) and relative paleointensity data sets. Alternative temporal trends in the paleomagnetic data were obtained by correcting for paleomagnetic lock-in depths between 0 and 70cm and taking into account changes in sediment accumulation rate. These temporal trends were regressed against reference curves for the same region (FENNOSTACK and FENNORPIS; Snowball et al., 2007). The best statistical matches to the reference curves are obtained when we apply lock-in depths of 21–34cm to the Gyltigesjön paleomagnetic data, although these are most likely minimum estimates. Our study suggests that a significant paleomagnetic lock-in depth can affect the acquisition of post-depositional remanent magnetization even where bioturbation is absent and no mixed sediment surface layer exists.

Synchronizing Holocene lacustrine and marine sediment records using paleomagnetic secular variation

High sediment accumulation rates in lacustrine and shallow-marine archives around Iceland offer the potential to compare high-resolution paleoclimatic reconstructions from terrestrial and marine archives; however, direct comparisons are hampered by difficulties in stratigraphic correlation and in deriving accurate age models for lacustrine archives. Icelandic paleomagnetic secular variation (PSV) has the potential to synchronize these records. Here we compare Holocene PSV from a well-dated marine core on the North Iceland shelf with PSV from two lacustrine archives with comparable sediment-accumulation rates, HVT03–1A, a glacier-dominated lake, and HAK03–1B, in a nonglacial catchment. Geochemically characterized tephra layers combined with unique high-amplitude structures in the PSV records provide secure tie points every ∼200 yr. Once the records are synchronized, the chronology from the marine core can be reliably transferred to the two lacustrine records. The resultant lacustrine age models reveal large changes in sediment accumulation rate at submillennial scales that escape detection in conventional age models with independent dates every ∼1 k.y. Sediment accumulation rate changes occur at similar times in both lakes, despite very different catchment properties. Low and regular accumulation rates during the Holocene thermal maximum suggest regionally stable, vegetated catchments, followed by a stepped landscape destabilization during the transition into neoglaciation, culminating with maximum sedimentation rates during the Little Ice Age. PSV allows synchronization between multiple records from nearby marine and lacustrine archives, providing improved age models and a means of assessing leads and lags between marine and terrestrial environments.

Natural and human impacts on centennial sediment accumulation patterns on the Umpqua River margin, Oregon

Abstract Quantifying patterns of sediment accumulation rates (SARs) over the past ~ 125 years on continental margins provides insight into diverse processes spanning the land–ocean boundary. In particular, temporal changes in the export of fluvial sediment can lead to changes in ocean margin SARs, whereas spatial patterns of accumulation reflect the net effect of wave-driven resuspension and current transport averaged over many years. To explore these issues we quantified SARs using 210 Pb geochronology at 73 stations on the shelf and upper slope off the Umpqua River, Oregon. Three types of 210 Pb profiles were observed: a well-defined roughly 10-cm-thick surface mixing layer (SML) underlain by a zone of logarithmic decrease on the distal mid to outer shelf and slope (type 1, n = 45); a deep (20–30-cm thick) SML underlain by a zone of logarithmic decrease in the sandy inner shelf (type 2, n = 8); and a composite profile with two distinct zones of logarithmic decrease below an ~ 10-cm-thick SML in a mid-shelf depocenter adjacent to the river mouth (type 3, n = 21). Type 3 profiles imply a 2–4-fold increase in the SAR that occurred, on average, in 1967 ± 13 y. Such an increase in SARs is consistent with the history of industrial logging in the Umpqua basin, which peaked in the two decades after World War II and coincided with a wet phase of the Pacific Decadal Oscillation (1944–1978) when average and peak river flows were elevated. Comparison of the Umpqua River shelf depocenter with the well-studied Columbia and Eel River systems reveals some important similarities and differences between these ‘marine-dispersal dominated’ systems, whereby forcing (e.g., river–ocean coherence) on the Umpqua is comparable to that of the Eel, whereas the spatial pattern is more similar to that of the Columbia. These seemingly paradoxical results can be reconciled by considering the relative significant wave height during periods of elevated sediment delivery.

Temporal constraints and pulsed Late Cenozoic deformation during the structural disruption of the active Kashi foreland, northwest China

In response to the ongoing Indo‐Asian collision, structural deformation has encroached into the Tian Shan foreland in western China since the early Miocene. In order to reconstruct a detailed history of foreland deformation along the southern margin of the Tian Shan, we have synthesized extensive mapping, analysis of seismic sections, magnetostratigraphy of the foreland fill and associated growth strata, apatite fission track dating, changes in sediment accumulation rates, and geodetic surveys of deformed fluvial terraces. Across an area exceeding 6500 km2 that spans the Kashi foreland in the northwestern Tarim Basin, we use these data to place temporal constraints on individual structures that delineate at least 4 distinct stages of deformation. Stage 1 involved initial uplift of hinterland structures beginning at 20–25 Ma. Stage 2 began at ∼16.3 Ma when the basement‐involved deformation front stepped south to the Kashi Basin Thrust that bounds the foredeep strata. Stage 3 occurred from 13.5 to 4 Ma, as the deformation front migrated south in episodic steps above thin‐skinned detachment and ramp‐flat folding. Finally, stage 4 began after 4 Ma with growth of the Keketamu Anticline followed by rapid southward migration of the deformation front to the Atushi and then Kashi detachment anticlines. On the basis of the deformed Tertiary strata and available seismic data, calculations of the minimum magnitude of shortening yield totals ranging from 10 to 32 km since the early Miocene, of which 7–12 km occurred since ∼4 Ma. Although average shortening rates since 16.3 Ma were 0.8–1.3 mm/a, rates were faster from 16.3 to 13.5 Ma (1.1–3.2 mm/a), after which they decreased to 0.5 mm/a between 13.5 and 4 Ma before increasing to 2.2–2.7 mm/a after 4 Ma. Similarly, the rate of southward propagation of the deformation front was initially 3–7 mm/a for the first 10 Ma of basin development, slowed to 1.5 mm/a between 13.5 and 4 Ma, and increased to more than 10 mm/a since 4 Ma. These changes in shortening and structural migration rates over time suggest that the loci of orogenic deformation shifted in and out of the foreland during the Neogene. Surveyed stream terraces suggest that, although most current deformation is located along the southern front of the foreland, modest deformation and shortening continue in the hinterland. Structural style within the foreland has also changed over time from imbricate thrust faults with hanging wall folds along the northern margin, to ramp‐flat faults and detachment folds along the southern margin. Where the basement rocks are previously deformed, such as in the hinterland of the Kashi foreland, thick‐skinned fault ramps and related fault propagation folds dominate the structural style. As syntectonic sedimentation within the foreland produced thick, subhorizontal stratigraphy, the structural style evolved to detachment and fault bend folding above these footwall flats within the Neogene strata.

Practical implications of quantifying ancient sedimentation rates in lakes

S ediments stored in lakes represent a valuable archive that can be used to reveal the erosion history of watersheds. A portion of the soil that is moved down gradient during runoff events is deposited in lakes, and the rate at which sediment accumulates should be proportional to the rate of erosion from the surrounding land. The ability to calculate historical sedimentation rates opens several opportunities for improved understanding and management of watershed processes. Examples include quantifying changes in erosion and deposition rates caused by anthropogenic or natural shifts in land use or by climate change. Erosion is difficult to quantify at the watershed scale because of the large degree of spatial and temporal variation in erosion rates. Changes in slope, vegetative cover, soil compaction and composition, and a host of other variables can result in a wide range of erosion rates over distances of a few meters. Erosion rates can likewise vary significantly with changes in rainfall intensity and time since the last precipitation event. Lakes receiving this variable influx have a normalizing effect that allows long-term changes in sediment accumulation rates, and thus changes in erosion at the watershed scale, to be identified. Changes in erosion at scales smaller …

Modelo bioestratigráfico cuantitativo para el terciario de la cuenca inferior del Madgalena, caribe colombiano.

The reinterpretation of biostratigraphic information by new models and quantitative correlation techniques substantially improves its resolution and its correlative potential, thus minimizing oil exploration risks. With this aim, biostratigraphic information, i.e. first (FO) and last (LO) occurrence events of benthonic and planktonic foraminifera was analysed from sixteen wells from the Lower Magdalena Valley (LMV). The inconsistencies found in the biostratigraphic record, i.e. varying successions of first and last appearances of species from well to well as a result of several factors such incomplete sampling and preservation, true variation in the distribution of fosil taxa, etc., and the great amount of biostratigraphic data makes it practically impossible to accurately constrain basin history from biostratigraphic information by unaided visual inspection. This motivates the treatment of biostratigraphic information with new quantitative approaches, such as constrained optimization (CONOP9 software) and graphic correlation concepts (GraphCor 3,0) and the comparative method approach of Cooper et al. (2001). The succession of biostratigraphic events found through the application of each technique was statistically filtered and compared with Kendall tau coefficients whose values were 0,8. An optimal biostratigraphic succession of LOs was found and calibrated with the Berggren et al. (1995) global time scale by a LOESS regression model for the middle Eocene-Pliocene interval, thus revealing three major changes in sediment accumulation rates for the basin during this time interval: (1) middle Eocene to Oligocene, with low accumulation rates, (2) early Miocene to middle Miocene, with high accumulation rates and (3) late Miocene to Pliocene, with lower accumulation rates. The calibrated composite succession enabled the construction of age-well depth plots, which indicate periods of local deposition and accumulation rates, and periods of erosion, no deposition or very low accumulation rates (unconformities). The best plots were used to build a model for the correlation of unconformities, which shows that they are heterochronous, lasted at least 2,5 Ma, and are of limited extent. Finally, a correlation model was proposed that includes: (1) a time-calibrated succession of biostratigraphic events, and (2) a Haq curve that shows how each geologic period is recorded in each well.

Past and future perspectives upon landscape instability in Cumbria, northwest England

Geomorphic and lacustrine evidence is used to identify phases of landscape instability in Cumbria, northwest England, British Isles. The temporal and spatial pattern of erosion is used alongside palaeovegetation data from across the region to elucidate the causes of landscape instability in the late Holocene. The lines of evidence include changes in sediment accumulation rate and provenance within the region’s lakes; the hillslope alluvial fan and gully incision record; the lowland fluvial geomorphic record; and pollen analytical data for the late Holocene vegetation history. The sediment supply driven hillslope and lacustrine records suggest that only the 1,200–800 BP and post 600–500 BP erosion episodes were both catchment-wide and directly affected upland hillslopes, with the earlier phases after 5,200 and 3,000–1,600 BP on a lower scale and more restricted spatially. These episodes coincide with major expansions or changes in anthropogenic activity during Neolithic, Iron Age and Romano-British, Norse and late Medieval times. The projected future trajectories of changes in both climate (UKCIP scenarios) and landuse (CAP reform and ESA status) appear unlikely to increase landscape instability, although a shift to greater incidence of storms and high magnitude flood events clearly could. The past shows that the largest increases in erosion and sediment movement occur in the wake of major intensifications in land pressure that primarily affect previously wooded or protected hillslopes, circumstances that land management strategists should mitigate against.

Paleolimnological investigations of anthropogenic environmental change in Lake Tanganyika: II. Geochronologies and mass sedimentation rates based on 14C and 210Pb data

We established sediment geochronologies for cores from eight deltaic areas in Lake Tanganyika (the Lubulungu, Kabesi, Halembe, Malagarasi, Nyasanga/Kahama, Mwamgongo, Nyamusenyi, and Karonge/Kirasa River deltas), recording a range of watershed disturbance histories from the eastern margin of this African rift valley lake. Cores from currently disturbed sites on the central Tanzanian coast display remarkably uniform and low rates of sediment accumulation from the 18th century until the early 1960s, when a synchronous and dramatic rise in rates occurs. Through this same time interval sedimentation rates offshore from undisturbed Tanzanian watersheds either remain unchanged or decline. Further north, at disturbed sites along the northern Tanzania and Burundi coasts, the pattern of sedimentation rate increase is more complex. Although a mid-late 20th century increase is also evident in these sites, indications of earlier periods of increasing sediment erosion, dating from the mid-late 19th century, are also evident. Synchronous changes in sediment accumulation rates dating from the early 1960s may be the result of exceptionally wet years triggering an increase in the discharge of previously eroded and unconsolidated alluvium and stream/beach terrace deposits, previously accumulated in the deltas and stream valleys of impacted watersheds. Sedimentation rate impacts of deforestation on lake ecosystems are likely modulated by short-term climatic forcing events, which can impact the specific timing and location of sediment discharge to lakes.