Investigation of faulting and abyssal hill formation on the flanks of the East Pacific Rise (21°N) using ALVIN

A wealth of geologic information has been collected during studies of the Matuyama/Brunhes magnetic reversal boundary on the East Pacific Rise at 21°N. Five ALVIN and two CYANA dives, and a series of deep-tow traverses show that abyssal hills in this region of the Pacific are created near the spreading axis by inward dipping normal faulting and by back-tilting of these fault blocks. Outward dipping faults occur but are of less importance in the creation of relief. Tectonic disruption of the crust, particularly through tilting, is less pronounced than in the Atlantic. Small volcanoes approximately 50 m high and 400 m wide are common on the abyssal hills. A significant number of the volcanoes may have split apart at the spreading axis attesting to the narrowness of the crustal accretion zone on the East Pacific Rise. Active faulting is restricted to less than 10 to 12 km off-axis, although minor recent faulting may have been detected 23 km off-axis. Crustal sections exposed by faulting reveal that massive lava flows and sheet flows are common in the upper portion of oceanic layer two, but are less abundant than pillow lavas.     

Energy dispersive X-ray analysis of the surface of a deep-sea ferromanganese nodule

In an effort to elucidate the mechanism of formation and accretion of ferromanganese nodules, transverse sections of undisturbed botryoid surfaces of nodules from the abyssal central North Pacific were examined by X-ray analyses both by multiple replicate sample transects and mapping. Surface concentrations of Mn and Fe were found of the size and shape of microorganisms. Abundant microorganism-like objects covering the surface and the apparent absence of Si (i.e. no sediment) with the Mn and Fe concentrations suggests microorganisms may be involved either in the selective accretion or removal of these metals. If metal accretion was simply physicochemical, one would expect a more or less even distribution of Mn and Fe on the botryoid surfaces.The replicate X-ray sample transects showed that Mn and Fe have a distribution corresponding to botryoid zonation. Areas of high Mn X-ray intensity were found associated with the cap and equatorial zones of the botryoid. Most of the valley zone of the botryoid show a more or less even Mn distribution and an increase in Fe X-ray intensity. At the base of the botryoid both metals apparently precipitously decrease.     

Quantitative evaluation of bioturbation rates in deep ocean sediments. II. Comparison of rates determined by 210Pb and 239,240Pu

Excess ²¹⁰Pb was determined in six abyssal cores from the Eastern Atlantic, Hatteras Abyssal Plain, and Puerto Rico Trench. A bioturbation rate for each core was evaluated using the mixing model of Guinasso and Schink. The rates were compared to mixing rates evaluated for the same cores using ^239,240Pu. In all cases, the excess ²¹⁰Pb-derived mixing rates were lower than the plutonium-derived mixing rates. ^239,240Pu was also present at greater depths than excess ²¹⁰Pb. One possible explanation for these observations is the association of plutonium with particles of greater food value which are bioturbated at faster rates.     

Accretion rates in brackish marshes of a Chesapeake Bay estuarine tributary

Vertical accretion rates were determined for brackish marshes in an estuarine tributary on the Eastern Shore of Chesapeake Bay. These rates determined on the basis of the peak phase of early European settlement as recorded in pollen spectra of the marsh sediments range between 0. 18 to 0.74 cm yr⁻¹ over approximately the last 194 years and generally decrease down the estuary. More recent accretion rates (estimated from changes in pine pollen concentration) appear to have accelerated, exceeding the present local rate of sea level rise. The implications for using estuarine marsh accretion rates as surrogates for changes in sea level are discussed.     

Sediment community oxygen consumption in the deep Gulf of Mexico

Sediment community oxygen consumption (SCOC) has been measured from the continental shelf out to the Sigsbee Abyssal Plain in the NE Gulf of Mexico (GoM). SCOC rates on the continental shelf were an order of magnitude higher than those on the adjacent continental slope (450–2750 m depth) and two orders of magnitude higher than those on the abyssal plain at depths of 3.4–3.65 km. Oxygen penetration depth into the sediment was inversely correlated with SCOC measured within incubation chambers, but rates of SCOC calculated from either the gradient of the [O₂] profiles or the total oxygen penetration depth were generally lower than those derived from chamber incubations. SCOC rates seaward of the continental shelf were lower than at equivalent depths on most continental margins where similar studies have been conducted, and this is presumed to be related to the relatively low rates of pelagic production in the GoM. The SCOC, however, was considerably higher than the input of organic detritus from the surface-water plankton estimated from surface-water pigment concentrations, suggesting that a significant fraction of the organic matter nourishing the deep GoM biota is imported laterally down slope from the continental margin.     

Numerical study of tide-induced mixing over rough bathymetry in the abyssal ocean

Locally enhanced turbulent mixing over rough bottom bathymetry is one of the candidates that might make up for the lack of diapycnal diffusivity in maintaining the global overturning circulation. In the present study, using a two-dimensional vertical numerical model for the Brazil Basin, we numerically examine the intensity and vertical structure of tide-induced mixing over multi-beam bottom bathymetry via the comparison with those over somewhat smoothed bottom bathymetry. Note that even this smoothed bottom bathymetry is finer than in commonly used datasets. In comparison to the response over the smoothed bottom bathymetry, energy dissipation rates are enhanced within a few hundred meters over the multi-beam bottom bathymetry. In spite of several limitations of the two-dimensional vertical numerical model, the magnitude and vertical distribution of the calculated dissipation rates agree well with those from microstructure measurements. We find that tidal interaction with fine-scale (≤2 km) bottom bathymetry efficiently generates high wavenumber internal waves, which are subject to local energy dissipation and hence strongly control the abyssal mixing; the most important finding is that the intensity and vertical decay scale of abyssal mixing are in a trade-off relationship with each other, which is not taken into account in the existing parameterizations.     

Surface evolution and carbon sequestration in disturbed and undisturbed wetland soils of the Hunter estuary,southeast Australia

The aim of this work was to quantify the soil carbon storage and sequestration rates of undisturbed natural wetlands and disturbed wetlands subject to restriction of tidal flow and subsequent rehabilitation in an Australian estuary. Disturbed and undisturbed estuarine wetlands of the Hunter estuary, New South Wales, Australia were selected as the study sites for this research. Vertical accretion rates of estuarine substrates were combined with soil carbon concentrations and bulk densities to determine the carbon store and carbon sequestration rates of the substrates tested. Relationships between estuary water level, soil evolution and vertical accretion were also examined. The carbon sequestration rate of undisturbed wetlands was lower (15% for mangrove and 55% for saltmarsh) than disturbed wetlands, but the carbon store was higher (65% for mangrove and 60% for saltmarsh). The increased carbon sequestration rate of the disturbed wetlands was driven by substantially higher rates of vertical accretion (95% for mangrove and 345% for saltmarsh). Estuarine wetland carbon stores were estimated at 700–1000 Gg C for the Hunter estuary and 3900–5600 Gg C for New South Wales. Vertical accretion and carbon sequestration rates of estuarine wetlands in the Hunter are at the lower end of the range reported in the literature. The comparatively high carbon sequestration rates reported for the disturbed wetlands in this study indicate that wetland rehabilitation has positive benefits for regulation of atmospheric carbon concentrations, in addition to more broadly accepted ecosystem services.     

Reconstructing pre-colonial oyster demographics in the Chesapeake Bay, USA

Recent estimates of growth and mortality rates in extant Chesapeake Bay, USA oyster (Crassostrea virginica) populations are used to quantify changes in both population abundance (dN/dT) and shell accretion (dS/dT) associated with modern population demographics. The demographics of oyster populations that would be required to maintain reef accretion rates commensurate with sea level rise over geological time frames are examined using estimates of oyster longevity in pre-colonial (pre -1600) times combined with parallel estimates of pre-disease endemic mortality. The analysis demonstrates that modern populations, with their disease related, age-truncated demographics, are generally not capable of maintaining and building biogenic reefs through accretion. Estimates of filtration rates associated with Chesapeake Bay oyster populations prior to 1600 considerably underestimate actual benthic-pelagic coupling during that period. Pristine oyster populations would have supported water column turnover rates on the order of minutes to hours. Thus, the spatial footprint of oyster reefs was limited by available productivity in the estuary. Accretion rate calculations for pristine (pre-1600) oyster reefs describe the intimate relationship between benthic-pelagic coupling and the presence or absence of oyster reefs and the associated communities.     

湛江湾填海工程对海床冲淤的影响预测

建立了湛江湾泥沙数学模型,模拟了湛江湾海床冲淤演变,模拟结果表明,湛江湾海床较为稳定,年冲淤强度小,模拟冲淤分布与历史海图资料得出的长期演变趋势符合。模型用于湛江湾围填工程对海床冲淤变化的影响预测,结果表明工程后海床冲淤强度没有明显的改变。另外,优选了围填方案,估计了港池的淤积强度。     

Contributions of mineral and organic components to tidal freshwater marsh accretion

Vertical accretion in tidal marshes is necessary to prevent submergence due to rising sea levels. Mineral materials may be more important in driving vertical accretion in tidal freshwater marshes, which are found near the heads of estuaries, than has been reported for salt marshes. Accretion rates for tidal freshwater marshes in North America and Europe (n = 76 data points) were compiled from the literature. Simple and multiple linear regression analyses revealed that both organic and mineral accumulations played a role in driving tidal freshwater marsh vertical accretion rates, although a unit mass of organic material contributed ∼4 times more to marsh volume than the same mass input of mineral material. Despite the higher mineral content of tidal freshwater marsh soils, this ability of organic matter to effectively hold water and air in interstitial spaces suggests that organic matter is responsible for 62% of marsh accretion, with the remaining 38% from mineral contributions. The organic material that helps to build marsh elevation is likely a combination of in situ production and organic materials that are deposited in association with mineral sediment particles. Regional differences between tidal freshwater marshes in the importance of organic vs. mineral contributions may reflect differences in sediment availability, climate, tidal range, rates of sea level rise, and local-scale factors such as site elevation and distance to tidal creeks. Differences in the importance of organic and mineral accumulations between tidal freshwater and salt marshes are likely due to a combination of factors, including sediment availability (e.g., proximity to upland sources and estuarine turbidity maxima) and the lability of freshwater vs. salt marsh plant production.     

Rates of coastal erosion and accretion in New Zealand

Abstract

Rates of coastal erosion and accretion for New Zealand are calculated for the period since early European colonisation. Methods used for calculating rates from cadastral plans, vertical aerial photographs and field measurements are described, evaluated, and illustrated with examples. The most natural reference line for measurements of shoreline changes and for defining the seaward boundary of land is the seaward limit of land vegetation. Measurements made from air photographs and plans at scales larger than 1:4000 have errors less than ± 1 m. As scales become small, errors increase proportionately. Along depositional shorelines, erosion and accretion generally occur at 0.5–4.0 m.y^?1. Maximum erosion and accretion rates are 25.4 m.y^?1 at North Kaipara Head and 68.9 m.y^?1 at Farewell Spit respectively. Cliff recession generally occurs at 0.25 ‐ 1.0 m.y^?1 with maximum rates of 2.25 m.y^?1 for mudstone cliffs at Cape Tumagain and 3.46 m.y^?1 for conglomerate cliffs at Ngapotiki.     

Basalts and serpentinite from the Puerto Rico Trench, 1. Petrology

Typical abyssal tholeiites occur on both north and south walls of the Puerto Rico Trench. Probably they were erupted in the Mid-Atlantic Ridge in Cretaceous time and afterwards were moved to the present position. In addition, doleritic basalts containing phenocrysts of pigeonite rimmed by augite were found from the north wall, and these rocks presumably represent a new type of abyssal tholeiites characterized by $\text{CaO}\text{Na}{}_2\text{O}$ ratios lower than those of ordinary abyssal tholeiites.A new analysis of serpentinite from the north wall is also recorded here.     

Side-Scan Sonar Characteristics and Manganese Nodule Abundance in the Clarion—Clipperton Fracture Zones,NE Equatorial Pacific

The deep ocean floor between the Clarion and Clipperton fracture zones (NE equatorial Pacific) has the highest known manganese nodule abundance in the world oceans. A detailed analysis of MR1 (Mapping Researcher 1, 11–12?kHz) sonar images and free-fall grab data in the Korean manganese nodule field areas reveals a close relationship between side-scan sonar characteristics of the seafloor and manganese nodule abundance. Eight sonar facies are identified based on back-scattering intensity and distribution patterns. These sonar facies can be interpreted as (1) volcanic seamounts (facies I-1), (2) bounding faults of abyssal hills (facies I-2 and II-1), (3) lava flows or volcanoclastic mass-flow deposits around the volcanic seamounts (facies I-3 and II-2), (4) crests of abyssal hills (facies II-1), (5) abyssal troughs between abyssal hills (facies III-1), (6) relatively flat areas (facies II-3 and III-2). In the areas where facies II-1 (abyssal hill crests with thin sediment cover) and II-3 (relatively flat areas draped by thin sediments) are dominant, manganese nodules occur abundantly. In contrast, zones comprising facies III-1 (abyssal troughs with thick sediment cover) and III-2 (relatively flat areas covered by thick sediments) are characterized by low abundance of manganese nodules. This relationship between distribution of sonar facies and manganese nodule abundance implies that (1) the qualitative difference in acoustic reflectivity of long-range side-scan sonar with some ground truth data is useful for regional assessment of manganese nodule occurrence over wide areas in a reasonable time, and (2) seafloor topography and sediment thickness are important controlling factors for regional occurrences of manganese nodules.     

渤海海峡区域现代沉积速率分布的数值计算

利用平面二维数值模型 ,计算了渤海海峡沉积速率的平面分布 ,计算结果与210Pb测年数据较为一致。渤海海峡中部泥区是现代泥质沉积区 (堆积速率<2mm/a) ,沉积速率分布趋势为南高北低 ;山东半岛北部近岸泥区的堆积速率在1mm/a以下。研究区西北部为海底冲刷区 ,其中渤海海峡北侧的老铁山水道附近为最强冲刷区 ,冲刷速率高达9mm/a。     

Side-Scan Sonar Characteristics and Manganese Nodule Abundance in the Clarion—Clipperton Fracture Zones, NE Equatorial Pacific

The deep ocean floor between the Clarion and Clipperton fracture zones (NE equatorial Pacific) has the highest known manganese nodule abundance in the world oceans. A detailed analysis of MR1 (Mapping Researcher 1, 11-12 kHz) sonar images and free-fall grab data in the Korean manganese nodule field areas reveals a close relationship between side-scan sonar characteristics of the seafloor and manganese nodule abundance. Eight sonar facies are identified based on back-scattering intensity and distribution patterns. These sonar facies can be interpreted as (1) volcanic seamounts (facies I-1), (2) bounding faults of abyssal hills (facies I-2 and II-1), (3) lava flows or volcanoclastic mass-flow deposits around the volcanic seamounts (facies I-3 and II-2), (4) crests of abyssal hills (facies II-1), (5) abyssal troughs between abyssal hills (facies III-1), (6) relatively flat areas (facies II-3 and III-2). In the areas where facies II-1 (abyssal hill crests with thin sediment cover) and II-3 (relatively flat areas draped by thin sediments) are dominant, manganese nodules occur abundantly. In contrast, zones comprising facies III-1 (abyssal troughs with thick sediment cover) and III-2 (relatively flat areas covered by thick sediments) are characterized by low abundance of manganese nodules. This relationship between distribution of sonar facies and manganese nodule abundance implies that (1) the qualitative difference in acoustic reflectivity of long-range side-scan sonar with some ground truth data is useful for regional assessment of manganese nodule occurrence over wide areas in a reasonable time, and (2) seafloor topography and sediment thickness are important controlling factors for regional occurrences of manganese nodules.     

一般高潮线与2 m等深线反映黄河三角洲冲淤变化的对比

黄河三角洲是我国乃至世界上陆地动态变化最快的地区之一,其冲淤变化备受关注.利用从遥感影像提取一般高潮线和从实测水深提取2 m等深线反映海岸冲淤的两种方法,对钓口河嘴、清水沟河嘴和清8河嘴分别进行了冲淤面积（淤积、蚀退和净造陆面积）的计算,并对两种方法得到的结果进行了对比.结果显示,两种方法相差较大,钓口河嘴、清水沟河嘴和清8河嘴,2 m等深线反映的冲淤面积（无论是淤积、蚀退还是净造陆面积）绝对值比一般高潮线反映的冲淤面积绝对值都明显偏大.在反映黄河三角洲冲淤变化上,遥感影像提取的一般高潮线可反映陆上三角洲的冲淤变化,实测水深则反映水下三角洲的冲淤变化.     

马里亚纳海沟万米级水文观测研究

深渊观测是开展深渊科学研究的前提。文章介绍了2020年7月"东方红3"船在马里亚纳海沟"挑战者深渊"附近完成的一次海洋调查。基于船载温盐深综合剖面测量系统获取的万米级剖面数据,分析全海深的温盐性质,并依据Thorpe尺度方法和细尺度参数化方法,进一步估算不同深度层的湍动能耗散率。结果表明:"挑战者深渊"的深层海水十分稳定, 3 000—5 000 dbar的温盐特征与下层绕极水相同;受弱层结背景下的内潮影响, 5 000—8 000 m的耗散率显著提升。本次调查获取的万米级水文剖面为马里亚纳海沟的深渊探索提供了数据方面的支撑。     

Analysis of a fast-spreading rise crest: The East Pacific Rise, 9° to 12° south

The axis of the East Pacific Rise is defined by a topographic block about 15 km wide and 300 to 350 m high which is flanked by abyssal hills 100 to 200 m high and 3 to 5 km wide. These hills often are tilted such that their steep slopes face the axis. An empirical model explaining these features combines axial extrusion to form the central block and rotational faulting to lower the shoulders of the axial block to the regional depth and tilt them outward.The axial block is offset about 10 km left-laterally at 10.0°S and a similar amount right-laterally at 11.5°S. Offsets (or lack of offsets) of young magnetic anomalies indicate that these axial displacements occurred between 1.7 and 0.9 m.y. ago and 0.7 m.y. ago and the present in the north and south. respectively. These small axial offsets are interpreted to be the result of either brief episodes of asymmetric see-floor spreading or discrete jumps in the site of spreading activity. Both axial shifts were to the west; a unidirectional sequence of such shifts occurring at the above rate of one per million years would be difficult to differentiate from true regional asymmetric spreading and might explain that phenomenon on other medium-to fast-spreading rises.Reconnaissance data from the east flank of the East Pacific Rise indicate that spreading activity began on that part of the rise between the 9°S and 13.5°S fracture zones approximately 8.2 m.y. ago when the site of crustal accretion jumped westward from the now dormant Galapagos Rise. Slope change in crust approximately 2 and 6 m.y. old imply faster spreading rates between about 6 and 2 m.y. ago than either before or after that time. Identification and correlation of anomaly 3 allows an estimate of about 90 mm/y for this higher east flank spreading rate. Since 1.7 m.y. ago spreading rates have averaged about 80 mm/y to the west and 77 mm/y to the east.     

连云港南部近岸带冲淤变化分析

选取连云港南部近岸带南北不同岸滩 4个典型断面的样品点进行主成分分析 ,得出研究区控制泥沙交换和冲淤变化的主要动力因素为潮流作用和潮流波浪混合作用。据此 ,尝试建立冲淤变化的协方差数学模型 ,经过对计算结果显著性分析和实证检验得出研究区域的冲淤趋势为“整体冲淤平衡 ,局部略有冲刷” ,从北到南 ,依次出现淤积平衡、冲淤平衡、冲刷平衡三种较为稳定的自然冲淤环境。     

Seasonal and annual variations in the volumetric sediment balance of a macro-tidal salt marsh

This paper examines the spatial and temporal variability in the volumetric sediment balance of Allen Creek marsh, a macro-tidal salt marsh in the Bay of Fundy. The volumetric balance was determined as the balance of inputs of sediments and organic matter via accretion on the marsh surface and outputs of sedimentary material primarily due to erosion of the marsh margin. Changes in marsh surface elevation were measured at 20 buried plates and 3 modified sediment elevation tables from 1996–2002, and detailed margin surveys were conducted in 1997, 1999 and 2001 using a differential global positioning system. Changes in surface area were calculated using GIS overlay analysis and used in conjunction with accretion and erosion data to derive volumetric estimates of gains and losses of sedimentary material in the marsh system.Currently the volumetric sediment balance at Allen Creek marsh is positive. However the processes of erosion and accretion demonstrate seasonal, annual and spatial variability. Inputs to the system include deposition on the marsh surface from sediment laden waters and from ice rafting of sediments. Sediment is deposited onto the marsh surface year round, even during the winter when vegetation cover is sparse, and the amount of deposition in general is not significantly correlated with the frequency of tidal inundations. Based on the data from 1996 to 2002, the mid and high marsh zones experience mean accretion rates of approximately 1.4 cm year^− 1 whereas accretion rates in the low marsh region are statistically significantly lower (0.8 cm year^− 1). The absolute amount of accretion varies between seasons and from year to year. The main loss to the marsh is through erosion of the marsh margin cliffs which can remove a comparatively large volume of sedimentary material in one mass wasting event and which also decreases the vegetated surface area available for deposition from sediment laden waters. The volume of material removed from the marsh margin almost tripled between 1997 (169 m³) and 2001 (502 m³) following breaching of the side of a tidal creek channel, altering the patterns of margin erosion and deposition in the marsh system. During this time, however, other sheltered areas of the marsh system, such as along the tidal creek banks, showed evidence of new vegetation growth, increasing the amount of vegetated surface area available for deposition.The processes of erosion and deposition on the marsh surface exhibit considerable spatial variability, with different regions of the marsh being more or less sensitive to seasonal variability in the dominant controls influencing sediment deposition and erosion in this system, namely wave activity, vegetation, ice and water depths. A key factor in predicting how a marsh will evolve and respond to a number of different controls, e.g. sea-level rise or reduced sediment supply, is to quantify both accretion of the marsh surface and erosion of the marsh margin, evaluating the marsh system as a volumetric whole. This study demonstrates that a marsh system should be assessed in three dimensions rather than simply as a surface of accumulation. This is particularly important for open coastal marshes exposed to the erosive action of waves.