Geoacoustic investigations of cold vents and sedimentary processes at the active continental margin offshore Nicaragua

Discharge of cold fluids from the seafloor into the ocean is known from many continental margins. The amount and mechanism of this fluid and volatile recycling are important factors for global mass balances. This work investigates cold vents and seeps in the forearc region of the active continental margin offshore Nicaragua with geoacoustic methods (multibeam bathymetry, deep-towed sidescan sonar, subbottom profiler). Ground truthing is available from camera system surveys and coring on several of the structures. The area is characterized by a high number of so-called mound structures visible in the multibeam bathymetry, and various intensity anomalies without a significant topographic expression but showing high backscatter in the sidescan sonar image. The increased backscatter on mounds and intensity anomalies is mainly related to an increased seafloor roughness, and attributed to hardgrounds like authigenic carbonate formations. A significant influence of the seafloor topography on the sidescan sonar signal is largely excluded. This is shown by a new processing algorithm, developed in this work. The detailed analysis of the seep sites suggests a classification into three end members and explains their formation through the interaction of several processes: Episodic fluid venting and associated authigenic carbonate formation, background sedimentation with subsidence, and erosion

Morpho-acoustic variability of cold seeps on the continental slope offshore Nicaragua: result of fluid flow interaction with sedimentary processes

Based on multibeam bathymetry, high-resolution deep-towed sidescan sonar and Chirp subbottom profiling 32 cold seep sites, already identified in Sahling et al. (2008a), have been studied in an approximately 1000 km2 large area ranging from 800 to 2600 m water depth along the middle slope of the active continental margin offshore Nicaragua. Ground truthing is available from towed camera surveys and coring on seven of the structures. The seeps occur in different settings on the slope: upslope and along the headwall of large submarine slides, as isolated eroded massifs, and forming linear ridges between deeply incised canyons. The seep sites show a wide range regarding their size and morphology, their backscatter intensity patterns, their structure in subbottom profiles, and their fluid venting activity inferred from seafloor observations. Surface extension of the seep sites ranges from less than 200 to more than 1500 m in diameter, and relief height varies between no relief and 180 m. Indications of extruded materials such as mud flows are not observed in the area of the seep sites. Instead the seeps are characterized by high proportions of authigenic carbonates. The carbonates occur as crusts, detritus, or single layers embedded in the seafloor sediments. They appear as high backscatter intensities on sidescan sonar images. On some seep sites living vent fauna indicative of active seepage is observed, but gas bubbles have not been observed. To explain the high morphological variability of the features, we propose a generic model including the interaction of several processes: (1) episodic fluid venting and associated authigenic carbonate formation; (2) background sedimentation and subsidence; (3) linear erosion along canyons and denudation on the slope surface

Pacific offshore record of plinian arc volcanism in Central America: 3. Application to forearc geology

[1] Sediment gravity cores collected on the Pacific slope and incoming plate offshore Central America reach up to 400 ka back in time and contain numerous ash layers from plinian eruptions at the Central American Volcanic Arc. The compositionally distinct widespread ash layers form a framework of marker horizons that allow us to stratigraphically correlate the sediment successions along and across the Middle America Trench. Moreover, ash layers correlated with 26 known eruptions on land provide absolute time lines through these successions. Having demonstrated the correlations in part 1, we here investigate implications for submarine sedimentary processes. Average accumulation rates of pelagic sediment packages constrained by bracketing tephras of known age range from ∼1–6 cm/ka on the incoming plate to 30–40 cm/ka on the continental slope. There are time intervals in which the apparent pelagic sedimentation rates significantly vary laterally both on the forearc and on the incoming plate where steady conditions are usually expected. A period of unsteadiness at 17–25 ka on the forearc coincides with a period of intense erosion on land probably triggered by tectonic processes. Unsteady conditions on the incoming plate are attributed to bend faulting across the outer rise triggering erosion and resedimentation. Extremely low apparent sedimentation rates at time intervals >50–80 ka suggest stronger tectonic activity than during younger times and indicate bend faulting is unsteady on a longer timescale. Submarine landslides are often associated with ash layers forming structurally weak zones used for detachment. Ash beds constrain ages of >60 ka, ∼19 ka, and <6 ka for three landslides offshore Nicaragua. Phases of intense fluid venting at mud mounds produce typical sediments around the mound that become covered by normal pelagic sediment during phases of weak or no activity. Using intercalated ash layers, we determine for the first time the durations (several hundred to 9000 years) of highly active periods in the multistage growth history of mud mounds offshore Central America, which is essential to understand general mud-mound dynamics

Mapping deep-water gas emissions with sidescan sonar

Emissions of methane gas from cold seeps on the seafloor have a strong impact on a number of biogeochemical processes. These processes include the development of deepsea benthic ecosystems via the process of anaerobic oxidation of methane [Boetius et al., 2000] or the precipitation of carbonates [Ritger et al., 1987]. The fluxes of other chemical species associated with methane emissions may even influence the chemical composition of seawater [Aloisi et al., 2004]. Such gas emissions may have been much more intensive in the past with a strong impact on global climate [Dickens, 1999], as suggested by carbon isotope data

Predicting spatial kelp abundance in shallow coastal waters using the acoustic ground discrimination system RoxAnn

Kelp forests represent a major habitat type in coastal waters worldwide and their structure and distribution is predicted to change due to global warming. Despite their ecological and economical importance, there is still a lack of reliable spatial information on their abundance and distribution. In recent years, various hydroacoustic mapping techniques for sublittoral environments evolved. However, in turbid coastal waters, such as off the island of Helgoland (Germany, North Sea), the kelp vegetation is present in shallow water depths normally excluded from hydroacoustic surveys. In this study, single beam survey data consisting of the two seafloor parameters roughness and hardness were obtained with RoxAnn from water depth between 2 and 18 m. Our primary aim was to reliably detect the kelp forest habitat with different densities and distinguish it from other vegetated zones. Five habitat classes were identified using underwater-video and were applied for classification of acoustic signatures. Subsequently, spatial prediction maps were produced via two classification approaches: Linear discriminant analysis (LDA) and manual classification routine (MC). LDA was able to distinguish dense kelp forest from other habitats (i.e. mixed seaweed vegetation, sand, and barren bedrock), but no variances in kelp density. In contrast, MC also provided information on medium dense kelp distribution which is characterized by intermediate roughness and hardness values evoked by reduced kelp abundances. The prediction maps reach accordance levels of 62% (LDA) and 68% (MC). The presence of vegetation (kelp and mixed seaweed vegetation) was determined with higher prediction abilities of 75% (LDA) and 76% (MC). Since the different habitat classes reveal acoustic signatures that strongly overlap, the manual classification method was more appropriate for separating different kelp forest densities and low-lying vegetation. It became evident that the occurrence of kelp in this area is not simply linked to water depth. Moreover, this study shows that the two seafloor parameters collected with RoxAnn are suitable indicators for the discrimination of different densely vegetated seafloor habitats in shallow environments

Interannual variability of sorted bedforms in the coastal German Bight (SE North Sea)

Sorted bedforms are ubiquitous on the inner continental shelves worldwide. They are described as spatially-grain-size-sorted features consisting of small rippled medium-to-coarse sand and can remain stable for decades. However, the knowledge about their genesis and development is still fragmentary. For this study, a representative investigation area (water depth <15 m) located on the shelf west of the island of Sylt (SE North Sea, Germany) was periodically surveyed with hydroacoustic means (i.e. sidescan sonar, multibeam echo sounder, and sub-bottom profiler) during 2010-2014. Since this area is influenced by tidal and wind-driven currents, the aim was to detect and examine interannual variabilities in the characteristics of the prevailing sorted bedforms. Our measurements reveal sinuous stripes of rippled medium sand which are embedded in shallow symmetrical depressions. These domains are surrounded by relatively smooth fine-sand areas. These sorted bedforms were identified as flow-transverse features that are maintained by ebb and flood currents of almost equal strengths that flow in opposite directions. This bidirectional flow field generates sharp boundaries between the medium- and fine-sand domains in both current directions. Further to the north, where flood currents are dominant, asymmetric sorted bedforms were detected which show sharp boundaries only in flood-current direction. Comparisons between the measurements of the different years show no significant variations in morphology and distribution of the sorted bedforms. However, variations of the boundaries between the medium and the fine-sand domains were observed. Additionally, new minor sorted bedforms and rippled excavation marks as well as new fine-sand areas developed and disappeared occasionally. It can be supposed that such sediment winnowing and focusing processes take place during periodically recurring storm surges, which change the shapes of the features. Moreover, variations in alignments and sizes of the small ripple formations were detected. They seem to indicate the directions and intensities of previous storm events

Detection of Kelp Vegetation off Helgoland (SE North Sea)Using the Acoustic Ground-Discrimination System RoxAnn

The seafloor of the SE North Sea is mostly characterized by unconsolidated sandy sediments of Tertiary, Pleistocene and early Holocene origin. However, Helgoland and the surrounding coastal waters are different. Hardrock ridges crop out and in between the ridges sandy to gravelly sediments occur. The rocky seafloor forms an ideal environment for macroalgae and associated organisms which dominate the vegetation along a depth gradient down to 10 - 12 m below sea chart. Kelps provide habitat and shelter for a wide variety of associated seaweeds and invertebrates and serve as natural coastal protection. Hence, observation of these habitats is of great interest. The aim of this study is (1) to determine the acoustic signatures of the kelp vegetation for automated mapping purposes using the single-beam seafloor-classification system RoxAnn (Model GD-X) and (2) to map the spatial distribution and variable densities of the kelp populations and other seaweed dominated communities within two areas in the North and in the South off Helgoland. The hydroacoustic survey was performed in June 2011 during times of high tide. Altogether 32 transects across the investigation area were recorded. RoxAnn works with a frequency of 200 kHz and measure hardness (soft to hard) and roughness (smooth to rough) properties of the seafloor as well as water depth. For positioning a Leica 1200 differential GPS was used. A Kongsberg underwater camera was utilized to ground-truth the acoustic data on 13 video transects. Additionally to the video transects ground truthing was achieved via georeferenced diving transects which provided detailed information on the percentage ground coverage of kelp species, red algae vegetation or other substrates. The whole acoustic data set consists of ~ 45,000 measurements. On the basis of the acoustic and ground-truth data different habitats were distinguished: Gravel fields and hardrock outcrops are characterized by high hardness parameters while sandy seafloor illustrates rather smooth signatures. However, the signature of kelp vegetation is rough and soft while a high density of the kelps correlates to high roughness value. Hence, the occurrence of vegetation was classified in four categories including (1) no, (2) sparse, (3) medium and (4) dense vegetation. Since RoxAnn only provides point data and no spatial information areas between the surveyed transects were interpolated. The resulting map reveals a number of small elongated kelp fields south off Helgoland Dune and two big dense kelp fields in the North as well as several smaller kelp accumulations. In conclusion, a rapid habitat mapping was carried out by hydroacoustic means showing an accurate spatial distribution of the kelp population. It was even possible to differentiate between the varying densities of the kelp fields