Excess 210Pb Research Articles

Changes of sedimentation environment inferred from fallout radionuclides and physical properties of sediment in Sun Moon Lake, Taiwan

This study describes the impact of dam construction at Sun Moon Lake, Taiwan, on lacustrine sedimentation. Analyses of the physical properties of sediment cores show that the surface sediment has two layers: the post-dam layer consisting of high-density, organic-poor sediment, and the underlying pre-dam layer consisting of organic-rich sediment. After correction for decay since initial deposition, the estimated concentration of excess 210Pb in the pre-dam layer was much higher than that in the present surface layer. These results indicate that the dam construction had a large impact on supply of 210Pb and the sedimentation environment in this lake.

A 210Pb chronological study in sediments from poços de caldas alkaline massif (PCAM), Brazil

The Constant Flux and Constant Sedimentation (CF:CS) of supported/excess 210Pb model was successfully used to study sediment profiles from Antas stream, located in the region of Poços de Caldas city, Minas Gerais State, Brazil. Historical changes in the region were tracked from evaluating the sedimentation rate by the 210Pb method. In that site, Osamu Utsumi mine was the first mining-industrial complex for the production of concentrated uranium in Brazil. Four sediment testimonies were sampled along Antas stream in order to determine sedimentation rates using 210Pb as geochronometer. 210Pb and 238U activity concentrations were determined in sediment samples by alpha spectrometry, allowing to find the excess 210Pb present in the sediments. Additionally, the main oxides, organic matter, particles size and water composition were determined in order to assist the results interpretation from radionuclides data. The results allowed find one (profile PKS-4) or two (profiles PKS-1, PKS-2 and PKS-3) sedimentation rates, probably due to changes in the sediments input regime in the region. The sedimentation rates were in the range between 0.26 and 0.94 g/cm2.year, corresponding to the interval of linear sedimentation rate of 0.21 – 0.92 cm/year. The deposition year in the bottom of PKS-4 profile as estimated from the sedimentation rate coincided with the construction year of Bortolan dam (1956). Large touristic interventions carried out at Poços de Caldas city from 1920s coupled to unbridled urbanization, industrialization and demographic growth there in the second half of the twentieth century possibly caused the changes found in the sedimentation rates.

Deposition patterns on the Chukchi shelf using radionuclide inventories in relation to surface sediment characteristics

Forty sediment cores collected on the Chukchi shelf and adjacent portions of the East Siberian Sea have been assayed for the sedimentation tracer 137Cs and 34 were also assayed for 210Pb. While both sedimentation and bioturbation influence how these tracers are distributed vertically in sediment cores, only about half of the cores had distinct, single mid-depth or subsurface maximum activity peaks associated with 137Cs originating from bomb fallout. For the same reasons, only 14 of the 34 cores assayed showed a consistent decline in excess sedimentary 210Pb with depth in the core. Furthermore, sedimentation rate estimates from 210Pb assays were only consistent with estimated 137Cs sedimentation rates in 4 of the 14 cores from north of Bering Strait. A high degree of bioturbation on the shelf is primarily responsible for these patterns, but the influence of sedimentation on vertical profiles is also important, particularly in areas of low accumulation where shallow burial of maximum burdens of 137Cs in high current areas such as Herald Canyon can be observed. Shallow burial of radiocesium is also observed in comparatively low sedimentation areas such as Hanna Shoal, on the northeast Chukchi Shelf. By contrast, elsewhere on the northeast Chukchi Shelf and in productive benthic “hotspots,” the stronger influence of bioturbation leads to radiocesium that is more evenly distributed vertically within sediments, i.e., no distinct mid- or subsurface depth maximum activity associated with bomb fallout. These sediment profiles of radiocesium reflect several other sediment characteristics that are affected by current flow. These sediment characteristics in turn impact biological activity, including grain size, carbon to nitrogen ratios of the organic fraction of surface sediments and total organic carbon content. The distribution patterns of the radionuclides, particularly the depth where 137Cs reaches maximum activity, reflects sedimentation under both quiescent and strong currents. The activity of 137Cs at that depth of maximum activity provide insights on how much the vertical distribution of the radionuclide has been impacted by bioturbation, as well as the characteristics of the sediments that play a role in influencing deposition and total inventories of radiocesium on continental shelves.

Historic carbon burial spike in an Amazon floodplain lake linked to riparian deforestation near Santarém, Brazil

Abstract. Forests along the Amazon Basin produce significant quantities of organic material, a portion of which is deposited in floodplain lakes. Deforestation in the watershed may then have potentially important effects on the carbon fluxes. In this study, a sediment core was extracted from an Amazon floodplain lake to examine the relationship between carbon burial and changing land cover and land use. Historical records from the 1930s and satellite data from the 1970s were used to calculate deforestation rates between 1930 to 1970 and 1970 to 2010 in four zones with different distances from the margins of the lake and its tributaries (100, 500, 1000 and 6000 m buffers). A sediment accumulation rate of ∼4 mm yr−1 for the previous ∼120 years was determined from the 240+239Pu signatures and the excess 210Pb method. The carbon burial rates ranged between 85 and 298 gCm-2yr-1, with pulses of high carbon burial in the 1950s, originating from the forest vegetation as indicated by δ13C and δ15N signatures. Our results revealed a potentially important spatial dependence of the organic carbon (OC) burial in Amazon lacustrine sediments in relation to deforestation rates in the catchment. These deforestation rates were more intense in the riparian vegetation (100 m buffer) during the period 1930 to 1970 and the larger open water areas (500, 1000 and 6000 m buffer) during 1970 to 2010. The continued removal of vegetation from the interior of the forest was not related to the peak of OC burial in the lake, but only the riparian deforestation which peaked during the 1950s. Therefore, this supports the conservation priority of riparian forests as an important management practice for Amazon flooded areas. Our findings suggest the importance of abrupt and temporary events in which some of the biomass released by deforestation, especially restricted to areas along open water edges, might reach the depositional environments in the floodplain of the Amazon Basin.

Historical accumulation of potentially toxic trace elements resulting from mining activities in estuarine salt marshes sediments of the Asturias coastline (northern Spain).

The extensive extraction activity of mercury ores in Asturias (northwest Spain), also rich in As and Sb, has impacted the Nalón river estuary. The objective of this research was to assess the historical evolution of As-Hg-Sb accumulation in the salt marsh sediments of this area. For this purpose, sediment cores were collected from two different salt marshes (eastern and western river banks) in the estuarine environment to evaluate the degree of anthropogenic enrichment and the geochronology of As-Hg-Sb accumulation. Core subsampling was performed by cutting 2-cm-thick slices of sediments. The subsamples were then analysed for several physical and chemical parameters. Sedimentation rate was assessed by measuring short-lived radionuclides (excess 210Pb and 137Cs). Pre-mining levels of As-Hg-Sb were observed at core depths below 50cm. In the less extended salt marsh (eastern river bank), maximum As-Hg-Sb concentrations of 87.48, 3.66, and 5.75μg·g-1, respectively, were found at the core top as a consequence of long-term mining activity in the area. The vertical distribution of As-Hg-Sb was influenced by the single-point contamination sources, whereas grain-size variability and diagenetic remobilisation did not seem affected. Geochronological measurements showed that the depositional fluxes of As-Hg-Sb were influenced by anthropogenic input after 1900, when mining activity in the area was most intense. Hg mining ceased in 1969; however, the corresponding core profiles did not show a drastic decreasing trend in element fluxes, implying that the river drainage basin retains some "memory" of contamination which affects riverine sediments. A preliminary gross estimation of total As-Hg-Sb "trapped" in the Nalón river salt marsh sediments amounted to approximately 18.7, 1.0, and 0.7t, respectively. These morphological structures suffer erosive processes, thus representing a potential source of these elements associated with sediments; consequently, management conservation and monitoring of salt marshes should be taken into consideration from this environmental point of view.

Variations in carbon burial and sediment accretion along a tidal creek in a Florida salt marsh

AbstractSalt marshes store large quantities of carbon in the form of buried organic matter (OM) and consequently play a major role in the global carbon cycle, yet vertical accretion and carbon burial rates (CBRs) can vary by orders of magnitude on small spatial scales. The goal of this study was to provide insight into carbon burial variability of a single tidal salt marsh. Six marsh sediment cores were collected along a tidal creek in the Big Bend of Florida from the mouth to the coastal forest within the marsh levee and plain. Each was analyzed for porosity, % OM, total organic carbon (TOC), total nitrogen (TN), δ13C, δ15N, and excess 210Pb to determine vertical accretion and CBRs. Porosity, % OM, and TOC and TN were found to be highest in the low marsh and within the marsh levee. Stable isotopes of OM indicate the source is dominated by C3 plant species in both the levee and plain. Average vertical accretion ranges from 0.9 mm yr−1 to 2.2 mm yr−1 with the slowest rates in the low marsh. Average carbon burial ranges from 49.5 g OC m−2 yr−1 to 109.5 g OC m−2 yr−1. High carbon burial associated with low sediment accumulation in the Low Marsh and low carbon burial associated with high sediment accumulation rates in the High Marsh are typical in this marsh. These variations imply that the highest carbon burial occurs in the zone most vulnerable to loss via inundation and erosion.

Sediment geochronology for bar-built estuaries subject to flood deposition and erosion: A robust multiproxy approach across an estuarine zone

This paper presents a contiguous late Anthropocene 210Pb sediment geochronology and a late Anthropocene flood/fire event geochronology for an estuary subject to flood deposition and erosion. The procedure used multiple proxies to identify and evaluate flood facies overlying regions of erosion. Reconstructions of the original baseline sediments were undertaken for the major 210Pb geochronological methods. Only the lower estuary core site, which was deficient in inventory of excess 210Pb of baseline sediments, over the expected atmospheric supply for, was chosen to calculate the total amount of erosion. Erosion, as both cumulative mass and thickness, was then proportioned across flood events, relative to the size of the Rp index flood signal. The Rp index is the product of a simple stepwise thermogravimetric procedure. The flood geochronology was constructed by correlating the relative magnitudes of pivotal floods from an existing 100-year rainfall/river flow model and correlations in peak sediment char content outside of the flood facies with contemporary fire records. Overall, it was determined that 210Pb sediment isotope tomography gave the closest convergence with flood and fire events over those of the constant rate of supply and constant initial concentration methods. The agreement signified that around 40% of the original baseline accretion (25 cm) had been eroded by floods over the last 83 years and that there was possibly considerably more erosion at the upper site, which led to the loss of two flood facies.

Pacific Proving Grounds radioisotope imprint in the Philippine Sea sediments

Radionuclide concentrations were studied in sediment cores taken at the continental slope of the Philippine Sea off Mindanao Island in the equatorial Western Pacific. High resolution deposition records of anthropogenic radionuclides were collected at this site. Excess 210Pb together with excess 228Th and anthropogenic radionuclides provided information about accumulation rates. Concentrations of Am and Pu isotopes were detected by gamma spectrometry, alpha spectrometry and ICP-MS. The Pu ratios indicate a high portion (minimum of 60%) of Pu from the Pacific Proving Grounds (PPG). This implies that the transport of PPG derived plutonium with the Mindanao Current southward is similarly effective as the previously known transport towards the north with the Kuroshio Current. The record is compared to other studies from northwest Pacific marginal seas and Lombok basin in the Indonesian Archipelago. The sediment core top was found to contain a 6 cm thick layer dominated by terrestrial organic matter, which was interpreted as a result of the 2012 Typhoon Pablo-related fast deposition.

Activation of gas bubble emissions indicated by the upward decreasing Lead-210 activity at a submarine mud volcano (TY1) offshore southwestern Taiwan

A submarine mud volcano (MV) known as TY1 is one of the largest conical structures found offshore southwestern Taiwan. Active gas bubble emissions at the flat crest of the mud volcano disperse sediment particles into the water column. Changes in the mud volcanism of TY1 were revealed by radiograph and grain size analysis of the sediment cores. 210Pb geochronology was applied to the near-seafloor sediments to reveal the detailed mud volcanism from the deposition rate. In a 58-cm long sediment core taken from the northern margin of the crest, known as TY1-N.170, the size and frequency of the mud clasts decrease upward. The clasts are smaller than 8mm above a sediment depth of 34cm below the sea floor (cmbsf). Sediments between 0 and 19 cmbsf are massive, and particles larger than 2mm are absent. An enrichment of coarse, silt-sized particles in the massive sediment unit and the restricted distribution of the unit suggest that the massive unit was generated by re-deposition of sediment particles that were dispersed into the water column by gas bubble emission. These characteristics suggest that during the last mud volcanism of TY1, there was a decrease in mud eruption energy, and gas bubble emission became the main activity. In core TY1-S.440, taken from the southern slope, a massive sediment unit enriched with coarse, silt-sized particles, is intercalated between mud breccia structures. This suggests repeated mud breccia flows caused by TY1. The excess 210Pb activity present in the massive sediment unit of TY1-N.170 suggests activation of gas bubble emission. In the massive unit, a decrease in excess 210Pb activity appears upward toward the seafloor. The highest value is 3.1dpm/g at 19.8cmbsf (26.4g/cm2 in cumulative mass); values lower than 1.0dpm/g are distributed 2.8–6.8 cmbsf (3.6–8.8g/cm2). The upwardly decreasing trend is opposite that of the reference core from which a reasonable areal sedimentation rate was obtained using a constant-flux constant-sedimentation model. The anomalous depth profile at the massive unit indicates an increasing rate of sediment deposition, which originated from the activation of gas bubble emission.

Determining rates of sediment accumulation on the Mekong shelf: Timescales, steady-state assumptions, and radiochemical tracers

Thirty-two kasten cores, collected from the proximal Mekong continental shelf, have been analyzed for their excess 210Pb distributions in an effort to establish rates of sediment accumulation over the past 100 years. The length of the cores varied from 0.5 to 3m, and stations sampled topset, foreset, and bottomset beds (water depths 7–21m). Apparent excess 210Pb sediment accumulation rates ranged from > 10cm/y (no down-core decrease of excess activity over 300cm core length) near the Song Hau river mouth, to 1–3cm/y in topset and foreset beds within 20–50km of the river mouth, to rates as low as 0.4cm/y in cores from bottomset beds. The 210Pb sediment accumulation rates yield an overall sediment burial rate of 6.1 × 1013g/y for the proximal deltaic deposits, which corresponds to 43% of the total modern Mekong sediment burial on the southern Vietnam shelf (1.4 × 1014g/y; based on our 210Pb and seismic data and 210Pb data from the literature). This shelf burial rate is in reasonable agreement with current long-term estimates of Mekong River sediment discharge (1.3–1.6 × 1014g/y) from the literature. The inventory of excess 210Pb in the proximal Mekong deltaic deposits indicates that the shoreward flow of offshore water (entrained during river/ocean mixing) is approximately twice the flow of the Mekong freshwater discharge. Organic-carbon 14C ages were measured on 10 cores from the proximal Mekong delta and compared to 210Pb sediment accumulation rates in the same core. The 210Pb accumulation rates in all 10 cores were considered to be more robust and accurate than the 14C geochronologies, primarily because of down-core variations in the source of organic carbon deposited on the seafloor (old terrestrial carbon versus younger marine carbon). Variations in the source of organic carbon accumulating in the seabed were resolved by measuring the δ13C value of the seabed organic carbon.

Constraining the Spatial Extent of Marine Oil Snow Sedimentation and Flocculent Accumulation Following the Deepwater Horizon Event Using an Excess 210Pb Flux Approach.

Following the Deepwater Horizon (DWH) event in 2010, there were several lines of evidence indicating the presence of marine oil snow sedimentation and flocculent accumulation (MOSSFA). A significant amount of marine oil snow formed in the water column of the northern Gulf of Mexico (nGoM), settled rapidly, and ultimately accumulated in the sediments of the nGoM. This study utilized a commonly used radioisotope tracer (excess 210Pb, 210Pbxs) from 32 sediment cores collected from 2010 to 2013 to characterize the spatial extent of MOSSFA on the seafloor. Relative to pre-DWH conditions, an increase in 210Pbxs flux occurred in two distinct regions: (1) in the western portion of the study area on an east-northeast to west-southwest axis, stretching 230 km southwest and 140 km northeast of the DWH wellhead, and (2) in the eastern portion of the study area on a 70 km northeast to southwest axis near the DeSoto Canyon. The total sedimentary spatial extent of MOSSFA, as calculated by increased 210Pbxs flux after 2010, ranged from 12 805 to 35 425 km2. 210Pbxs flux provides a valuable tool for documenting the spatial extent of MOSSFA following DWH and will continue to aid in the determination of advective transport and ultimate depocenters of MOSSFA material.

Changes in organic carbon accumulation driven by mangrove expansion and deforestation in a New Zealand estuary

Mangroves are rapidly being lost to deforestation in many locations while expanding their areal extent in other subtropical and temperate regions. Currently, there is a paucity of information on how these changes may alter the carbon accumulation capacity of coastal areas. Here, sediment cores were collected from two areas and used to determine the influence of mangrove migration and deforestation on sediment carbon stocks and accumulation rates. The deforested area contained lower sedimentary organic carbon stocks (2767 ± 580 g m−2) compared to the preserved area (6949 ± 84 g m−2). Sediment accumulation rates, derived from excess 210Pb and 239+240Pu depositional signatures, ranged from 0.19 to 0.35 cm yr−1. The total sedimentary organic carbon (TOC) accumulation rates for the period after mangrove deforestation (2005–2011) exhibited significant differences between preserved areas (Core C: 43.9 ± 6.9 g m−2 yr−1; Core D: 83.1 ± 5.9 g m−2 yr−1) and the deforested area (Core B: 25.8 ± 6.0 g m−2 yr−1), suggesting a decline after deforestation. For the preserved area, the TOC accumulation under mangrove dominance (65.5 ± 16.3 g m−2 yr−1, after 1944) was higher than under saltmarsh dominance (23.5 ± 15.9 g m−2 yr−1, before 1944), as revealed by carbon isotopic signatures (δ13C). The increase in the TOC accumulation due to mangrove expansion in this New Zealand estuary was conservatively estimated as three-fold higher, and two-fold higher in stocks in comparison to the period when this ecosystem was dominated by non-mangrove vegetation.

Assessment of gamma radiation levels and natural radioactivity in soils along a subtropical river basin, China

Abstract The activities of natural radionuclides in the environment can be used to assess radiological effects. Monitoring the radiation level in soils is important for public health. It also has important geochemical implications as most of the sediment eroded from river basins is from soil. Therefore, we carried out a soil sampling campaign along a subtropical river basin in southeastern China (Jiulong River). Surface and depth profile soils were collected, and the natural radionuclide activities were measured. The activities of the natural radionuclides 238U, 232Th, and 40K in the surface soils varied from 31.6 to 132.1 Bq kg-dry−1, 37.8 to 174.0 Bq kg-dry−1, and 52.3 to 596.2 Bq kg-dry−1, with average values of 56.7±30.3 Bq kg-dry−1, 86.7±41.3 Bq kg-dry−1, and 352.8±190.6 Bq kg-dry−1, respectively. The absorbed gamma dose in air and the annual effective dose equivalent (AEDE) in surface soils along the river basin were both higher than the world average. In the depth profiles, excess 210Pb (210Pbex) decreased with depth and significant correlation between 210Pbex and TOC was observed, suggesting that they are affected by similar processes (leaching and sorption).

Historical changes in 239Pu and 240Pu sources in sedimentary records in the East China Sea: Implications for provenance and transportation

Concentrations and isotopic compositions of plutonium (Pu) are widely used for its source identification and to determine transport processes of Pu-associated particulate matter and water. We investigated the concentrations of 239Pu and 240Pu and their ratios in a number of sediment samples from the East China Sea (ECS) collected in the summer of 2013 (August 6–28). The 239+240Pu activity concentrations in surface sediment samples were found to range between 0.048 and 0.492 Bq kg−1 and the 240Pu/239Pu atom ratios showed a similar trend as that of the 239, 240Pu activities; the Pu atom ratios ranged from 0.158 to 0.297 and were mostly higher than the mean global fallout value of 0.18. The 239, 240Pu inventories in the ECS varied widely, from 2 to 807 Bq m−2, with the highest values commonly found in the coastal areas. In the Yangtze Estuary, the mean 239+240Pu activity concentration is close to the estimated value of the suspended material from the Yangtze River catchment (0.18 Bq kg−1), and the 240Pu/239Pu atom ratio was found to be ∼0.18, which indicates that the Yangtze River input is the dominant source of Pu for this area. The total annual Yangtze River input of 239+240Pu was estimated to be 2.4×1010 Bq, which is small compared to the total amount of 239+240Pu buried, 3.1×1013 Bq in the whole ECS. The Pacific Proving Ground input appears to be the dominant source of Pu to the ECS, accounting for 45%–52% of the total inventory. The fractional amount of 239+240Pu scavenged from the total 239+240Pu transported by the Kuroshio Current (KC) and Taiwan Warm Current (TWC) into ECS sediments is estimated to be ∼10%. Our study shows that the 240Pu/239Pu atom ratio is useful not only to obtain a better insight of the biogeochemistry influenced by the KC, but also to trace the long-range transport of other particle-reactive species. Besides, the sedimentation rates obtained based on the penetration depths of 239+240Pu and vertical profiles of excess 210Pb agree within uncertainties, which suggests that 239+240Pu can potentially be used as a chronostratigraphic time marker in the marine environment.

Provenances, distribution, and accumulation of organic matter in the southern Mariana Trench rim and slope: Implication for carbon cycle and burial in hadal trenches

It is generally accepted that organic matter content and accumulation rate at the seafloor attenuate with increasing water depth. However, hadal trenches, which represent the deepest portion of the hydrosphere on Earth, do not abide by this universal rule. It has been speculated that hadal trenches would serve as organic matter depocenters, where bacteria-mediated organic matter degradation intensifies. Here we examine the contents of total organic carbon (TOC) and total nitrogen (TN), as well as the δ13C values of TOC in three gravity cores, two box cores, and three grab samples with water depth from 4900 to 7118m, in order to reveal the provenances of organic matter, spatial distribution and accumulation rates of particulate organic carbon (POC) in the southern Mariana Trench rim and slope. Although the deepest area was not sampled, trench rim and slope is also of importance in terms of transportation and accumulation processes of organic matter. A vast majority of the sediment samples have bulk TOC/TN molar ratios and δ13C values of TOC ranging from 4.2 to 11 and from −21.8 to −18.9‰, respectively, implying that the organic matter was primarily sourced from marine algae. Two exceptions have been found at 101cm and 201cm depths of core GC05 with a possible input of terrestrial material suggested by an abrupt increase in TOC contents and TOC/TN molar ratios accompanied by marked decreases in δ13C values of TOC. Moreover, TOC contents in surface sediments basically increased with water depth. Based on the published excess 210Pb data and by assuming a simplified one-dimensional sediment loading despite of frequent occurrences of mass wasting transports in hadal trenches, the sedimentation rate in the southern Challenger Deep (6037m) was estimated to be 0.02cmyr−1. This much higher sedimentation rate compared to the globally averaged values in deep ocean sediments coupled with the trend of increasing in TOC contents with water depth may serve as evidence for the lateral transport of sediment particles induced by the funnel-like topography and localized current dynamics within the trench. The average POC accumulation rate within the southern Challenger Deep roughly amounts to 1.5×10−5gcm−2yr−1, equivalent to about a seventh of the globally averaged POC accumulation rate in deep-ocean seafloor. To our knowledge, POC accumulation in hadal trenches has been quantified for the first time, and we highlight that organic matter degradation in deep hadal trenches should not be neglected and may represent a significant component of the global carbon cycle.

Influence of deposition in dam reservoir on the deep marine hemipelagic environment off Niigata, central Japan

It has been well known that dam construction on rivers can cause a decrease in sediment supply and serious coastal erosion, while influence of dams on the sedimentary environment beyond the coastal area has not been well known. The mass accumulation rate (MAR) in the hemipelagic environment (500–700 m water depth) off Niigata, central Japan since the early half of the twentieth century was obtained from core samples, based on excess 210Pb activity. Decreases in MAR during the mid-twentieth century were recognized in several core samples located northern off the Agano and Shinano river mouths. Using the CIC model, combining correction for the influence of surface bioturbation based on the ratio of MARs, decreases in MAR were estimated as occurring during 1959–1964 and 1949–1955. These dates correspond with construction of the Okutadami dam (1956–1961), possessing the largest capacity and reservoir sediment volume of 3.9 × 107 m3 in the Agano and Shinano river systems. The other core located NW off the Agano river mouth showed constant MAR through the mid-twentieth century. A synchronicity of huge dam construction and decrease in MAR in deep marine environment, the similarity between silt–clay-sized grain size of dam reservoir sediment and that seen in the hemipelagic environment, and horizontal distribution of decreasing MAR in cores according to north–northeastward sediment transport on shelf, makes it reasonable to interpret that the decrease in MAR in hemipelagic environment was caused by entrapment of silt–clay grains in the huge dam reservoir.

Disentangling Natural and Anthropogenic Signals in Lacustrine Records: An Example from the Ilan Plain, NE Taiwan

The impact of human activities has been increasing to a degree where humans now outcompete many natural processes. When interpreting environmental and climatic changes recorded in natural archives on historical time scales, it is therefore important to be able to disentangle the relative contribution of natural and anthropogenic processes. Lake Meihua on the Ilan Plain in northeastern Taiwan offers a particularly suitable opportunity to test how human activities known from historical records can be recorded in lacustrine sediment. For this purpose, three cores from Lake Meihua have been studied by a multiproxy approach, providing the first decadal-resolution lacustrine records covering the past 150 years in Taiwan. Profiles of excess 210Pb, 137Cs and 239,240Pu from two short cores (MHL-09-01 and MHL-11-02) allowed a precise chronology to be established. The presence of a yellow, earthy layer with lower levels of organic material coincide with the record of land development associated with the construction of the San-Chin-Gong Temple during AD 1970-1982. Furthermore, in the lower part of the cores, the upwards increasing trend of inc/coh, TOC, TOC/TN, and grain size, coupled with the palynological data (increase of Alnus, Mallotus, Trema and herbs) from the nearby core MHL-5A with radiocarbon chronology, suggest that the area surrounding the lake has been significantly affected by agricultural activities since the arrival of Chinese settlers around ~AD 1874. In sum, this study demonstrates that this suite of lacustrine sediments in northeastern Taiwan has recorded human activities in agreement with historical documents, and that different human activities will leave distinct sedimentological, geochemical, and palynological signatures in the sedimentary archives. Therefore, multiproxy reconstructions are important to capture the complex nature of human-environmental interactions. A better understanding of the weathering and erosion response to human activities can also provide useful information for sustainable land-use management.

Earthquake-triggered deposits in the subduction trench of the north Ecuador/south Colombia margin and their implication for paleoseismology

The north Ecuador/south Colombia convergent margin is affected by recurrent subduction earthquakes with magnitudes >7.5, like the 1906, 1942, 1958, 1979 and 2016 events. The subduction trench is characterized by the construction of the Esmeraldas Turbidite System (ETS) fed by the large Esmeraldas Canyon that deeply incises the continental slope and that connects directly onshore with the Esmeraldas River. The detailed description of cores collected in the left-hand (western) proximal levee of the ETS and in two lobes allowed discriminating two types of coarse-grained deposits: (1) “classical” flood-generated turbidites are normally graded beds with structureless, laminated and cross-laminated intervals and high organic-matter content, while (2) earthquake-induced deposits consist of amalgamated normally-graded laminated/cross-laminated intervals separated by erosive surfaces. These latter are interpreted to be deposited by quasi-synchronous flows generated during a single earthquake. Organic matter is absent in such beds while ferromagnesian minerals and pumices are abundant, suggesting remobilization of the slope deposits. When two amalgamated beds are superimposed, the interbedded clayey interval is not bioturbated, suggesting a short time period between the beds deposition, and thus the impact of a major earthquake shock and following earthquakes on the triggering of landslides.Along the ETS, core-to-core correlation based on 210Pb excess revealed that 20th Century sedimentation occurred mainly in the proximal levee. There, a temporal relationship was established between the 1906, 1942, and 1979 earthquakes, and three coarse-grained beds showing features of earthquake-induced turbidites, suggesting the Esmeraldas Canyon was the main source for sediments to be remobilized during these earthquakes. The fining and thinning observed between the 1906, 1942 and 1979 turbidites correlate with the increasing distance of the rupture zone of each earthquake with the Esmeraldas Canyon. Earthquakes with magnitudes lower than 7 also affected the margin during the 20th Century but were not recorded in the trench sedimentation, suggesting that the turbidite levee acts as a natural filter so that potentially the highest the levee the strongest the earthquake magnitude recorded. At least ten earthquakes with the highest magnitudes were recorded on the turbidite levee within the last 800years with a recurrence time ranging from about 268years to 42–82years, or less for the 20th Century earthquakes. The comparison of the main features of the 1906 turbidite with older earthquake-triggered turbidites identified in a core collected in the trench suggests that one or two earthquakes similar to the 1906 event might have occurred ~600years ago.

Inconsistencies between 14C and short-lived radionuclides-based sediment accumulation rates: Effects of long-term remineralization

14C is the most widely utilized geochronometer to investigate geological, geochemical and geophysical problems over the past 5 decades. Establishment of precise sedimentation rates is crucial for the reconstruction of paleo-climate, -ecological and – environmental studies when extrapolation of sedimentation rates is utilized for time scales beyond the dating range. However, agreement between short-term and long-term sedimentation rates in anthropogenically unperturbed sediment cores has not been shown. Here we show that the AMS 14C-based long-term mass accumulation rate (MAR) of an organic-rich (>70%) sediment core from Mud Lake, Florida to be ∼5 times lower than the short-term MAR obtained using 239,240Pu, 137Cs and excess 210Pb (210Pbxs). The measured sediment inventories of 210Pbxs, 137Cs and 239,240Pu are comparable to the atmospheric fallout for the sampling site, indicating very little accelerated sediment erosion over the past several decades. Presence of sharp fallout peaks of 239,240Pu indicates very little sediment mixing. The penetration depths of 137Cs and 239,240Pu were found to be much deeper than expected and this is attributed to their post-depositional mobility. MAR calculated using 14C-ages in successive layers also indicated decreasing MARs with depth, and was reflective of progressive remineralization. Using first-order kinetics, the sediment remineralization rate was found to be 4.4 × 10−4 y−1 and propose that over the long-term, remineralization of organic-rich sediment affected the long-term MAR, but not the ratio of 14C/12C. Thus, the MAR and linear sedimentation rate obtained using 14C (and other isotope-based methods) could be erroneous, although 14C ages may not be affected by such remineralization. Long-term remineralization rates of organic matter has a direct bearing on the biogeochemical cycling of elements in aqueous systems and mass balance of elements needs to be taken into consideration.

Benthic carbon mineralization in hadal trenches: Assessment by in situ O2 microprofile measurements

Hadal trenches are considered to act as depo-centers for organic material at the trench axis and host unique and elevated biomasses of living organisms as compared to adjacent abyssal plains. To explore the diagenetic activity in hadal trench environments we quantified in situ benthic O2 consumption rates and sediment characteristics from the trench axis of two contrasting trench systems in the Pacific Ocean; the Izu-Bonin Trench underlying mesotrophic waters and the Tonga Trench underlying oligotrophic waters. In situ oxygen consumption at the Izu-Bonin Trench axis site (9200m; 746±103µmolm−2d−1; n=27) was 3-times higher than at the Tonga Trench axis site (10800 m; 225±50µmolm−2d−1; n=7) presumably reflecting the higher surface water productivity in the Northern Pacific. Comparing benthic O2 consumption rates measured in the central hadal Tonga Trench to that of nearby (60km distance) abyssal settings (6250m; 92±44µmolm−2d−1; n=16) revealed a 2.5 higher activity at the trench bottom. Onboard investigations on recovered sediment furthermore revealed that the prokaryotic abundance and concentrations of phytopigments followed this overall trend (i.e minimum values at the abyssal site followed by higher values from the Tonga and Izu-Bonin Trenches axis, respectively). Excess 210Pb profiles suggested that mass-wasting events contributed to the deposition of material enhancing the concentration of organic matter in the central trench as compared to the abyssal settings. Our results complement recent findings from the Challenger deep in the Mariana Trench area, which also revealed elevated diagenetic activity in the central trench underpinning the importance of hadal ecosystems for the deep sea carbon cycling.