ALBERT

Description: We use data from an extensive multibeam bathymetry survey of the Chile Ridge to study tectonomagmatic processes at the ridge axis. Specifically, we investigate how abyssal hills evolve from axial faults, how variations in magmatic extension influence morphology and faulting along the spreading axis, and how these variations correlate with ridge segmentation. The bathymetry data are used to estimate the fraction of plate separation accommodated by normal faulting, and the remaining fraction of extension, M , is attributed primarily to magmatic accretion. Results show that M ranges from 0.85 to 0.96, systematically increasing from first- and second-order ridge segment offsets towards segment centers as the depth of ridge axis shoals relative to the flanking highs of the axial valley. Fault spacing, however, does not correlate with ridge geometry, morphology, or M along the Chile Ridge, which suggests the observed increase in tectonic strain toward segment ends is achieved through increased slip on approximately equally spaced faults. Variations in M along the segments follow variations in petrologic indicators of mantle melt fraction, both showing a preferred length scale of 50+/-20 km that persists even along much longer ridge segments. In comparison, mean M and axial relief fail to show significant correlations with distance offsetting the segments. These two findings suggest a form of magmatic segmentation that is partially decoupled from the geometry of the plate boundary. We hypothesize this magmatic segmentation arises from cells of buoyantly upwelling mantle that influence tectonic segmentation from the mantle, up. This article is protected by copyright. All rights reserved.

Description: In order to improve robustness of remote sensing algorithms for lakes, it is vital to understand the variability of inherent optical properties (IOPs) and their mass-specific representations (SIOPs). In this study, absorption coefficients for particulate and dissolved constituents were measured at 38 stations distributed over a biogeochemical gradient in Lake Balaton, Hungary. There was a large range of phytoplankton absorption ( a ph (λ)) over blue and red wavelengths ( a ph (440)=0.11-4.39 m −1 , a ph (675)=0.048-2.52 m −1 ), while there was less variability in chlorophyll-specific phytoplankton absorption ( a* ph (λ)) in the lake ( a* ph (440)=0.022±0.0046 m 2 mg −1 , a* ph (675)=0.010±0.0020 m 2 mg −1 ) and adjoining wetland system, Kis-Balaton ( a* ph (440)=0.017±0.0015 m 2 mg −1 , a* ph (675)=0.0088±0.0017 m 2 mg −1 ). However, in the UV, a* ph (350) significantly increased with increasing distance from the main inflow (River Zala). This was likely due to variable production of photoprotective pigments (e.g. MAAs) in response to the decreasing gradient of colored dissolved organic matter (CDOM). The slope of CDOM absorption ( S CDOM ) also increased from west to east due to larger terrestrial CDOM input in the western basins. Absorption by non-algal particles ( a NAP (λ)) was highly influenced by inorganic particulates, as a result of the largely mineral sediments in Balaton. The relative contributions to the absorption budget varied more widely than oceans with a greater contribution from NAP (up to 30%), and wind speed affected the proportion attributed to NAP, phytoplankton or CDOM. Ultimately, these data provide knowledge of the heterogeneity of (S)IOPs in Lake Balaton, suggesting the full range of variability must be considered for future improvement of analytical algorithms for constituent retrieval in inland waters. This article is protected by copyright. All rights reserved.

Description: Phytoplankton chlorophyll-a (Chl) seasonal cycles of the North Atlantic are described using satellite ocean color observations covering the 1980s and the 2000s. The study region is where warmer SST and higher Chl in the 2000s as compared to the 1980s have been reported. It covers latitudes from 30°N–50°N and longitudes from 60°W–0°W, where two phytoplankton blooms take place: a spring bloom that follows stratification of upper layers, and a fall bloom due to nutrient entrainment through deepening of the mixed layer. In the 1980s, spring and fall blooms were of similar amplitude over the entire study region. In the 2000s, the fall bloom was weaker in the eastern Atlantic (east of 40°W), because of a delayed deepening of the mixed layer at the end of summer (mixed layer depth (MLD) determined from in situ data). Conversely, the spring bloom of the eastern Atlantic was stronger in the 2000s than it was in the 1980s, because of a deeper MLD and stronger winds in winter. In the Northwestern Atlantic (northwest of 38°N–40°W), little differences are observed for spring and fall blooms, and for the wintertime MLD. Our results show that the links between upper layer stratification, SST changes, and biological responses are more complex than the simple paradigm that sequentially relates higher stratification with warmer SST and an enhanced (weakened) growth of the phytoplankton population in the subpolar (subtropical) region.

Description: Determination of the relative inputs of aquatic autochthonous and terrestrial allochthonous organic matter into marine and lacustrine environments is essential to understanding the global carbon budget. A variety of proxies are used for this purpose, including the Branched and Isoprenoid Tetraether (BIT) index. This is calculated from the concentrations of branched glycerol dialkyl glycerol tetraethers (GDGTs), derived from unidentified terrestrial bacteria, and crenarchaeol, a marker for aquatic mesophile Thaumarchaeota (Crenarchaeota group I). As the index is a ratio, its value depends on both the crenarchaeol aquatic in situ production and the soil-derived branched GDGT input. Therefore, the BIT index reflects not only changes in the input of terrestrial or soil organic matter but also relative variations in aquatic Thaumarchaeota abundance in the water column. In fact, we show that in oceanic and lacustrine settings, the BIT index can be dominated by the aquatic end-member of the ratio. Consequently, the BIT index by itself can be an unreliable proxy to compare the input of terrestrial matter between sites and over time, and we propose that the quantification of branched GDGT fluxes or concentrations may instead be a better indicator of soil terrestrial inputs.

Description: Recent work across the Mediterranean Sea has illustrated the salinity and overgrowth effects on planktonic foraminiferal Mg/Ca, which potentially confound the use of this as a temperature proxy for paleoceanographic reconstructions. To test and verify these effects, we present new Aegean Sea results which reveal Mg/Ca values that were unreasonably high to be explained by temperature or salinity variations alone, confirming that foraminiferal Mg/Ca is affected by diagenesis. We have specifically targeted Globigerinoides ruber (w, sensu stricto), from a series of modern core tops spanning a strong sea surface salinity gradient and a minor sea surface temperature range, along a north-south Aegean Sea transect. Scanning Electron Microscopy analyses show that G. ruber specimens were covered by microscale euhedral crystallites of inorganic precipitates. This secondary calcite phase seems to be responsible for the anomalously high Mg/Ca ratios and likely formed near the sediment/water interface from CaCO3 supersaturated interstitial seawater. We also have clear evidence of diagenetic alteration in a north-south direction along the Aegean Sea, possibly depending on salinity and calcite saturation state gradients. These observations illustrate the necessity of alternative techniques (e.g., flow-through time resolved analysis or laser ablation inductively coupled plasma mass spectrometry) to potentially overcome these diagenetic issues and develop a more reliable and sensitive temperature proxy in similar subtropical settings characterized by high salinity, excessive evaporation, and restricted circulation.

Description: High spatial resolution hydrographic data, including Lowered Acoustic Doppler Current Profiler (LADCP) measurements, were acquired along a meridional section at 24.5°W in October 2009. The data are analyzed in detail with the purpose of definitively defining and quantifying the zonal Azores Current System. The Azores Current and Azores Countercurrent are delimited, each extending meridionally for 110 km. The Azores Current is located between 33.5°N and 34.5°N, flanked to the north by the Azores Countercurrent (35.25°–36.25°N). Vertically, both currents reach the γn = 27.975 kg m−3 level (∼2000 m depth), their mass transports ranging across thermocline as well as intermediate layers. The Azores Current transports 13.9 Sv (1 Sv = 106 m3 s−1 ≈ 109 kg s−1) eastward with its maximum associated with the Azores Front (33.75°N). The Azores Countercurrent flows below the surface, transporting 5.5 Sv westward. This contributes to a net eastward flow of 8.4 Sv across the section. At intermediate layers, the Azores Countercurrent transports mixed Mediterranean Water to the west, and the Azores Current transports mixed Sub-Arctic Intermediate Water to the east. Shipboard ADCP and satellite-derived geostrophic velocity are used to confirm the transports revealed by the hydrographic data.

Description: We present a new detailed tectonic model of the Reykjanes Ridge which examines the rift propagation hypothesis for the V-shaped ridges and its asymmetric lithospheric accretion. Four major southward rift propagations extend through our entire survey area and several additional small scale rift propagations are observed, including northward propagators. If plume pulses only drive southward propagators, then two different driving mechanisms for propagators must exist. There is a major difference in the crustal accretion asymmetry between the area immediately off the Iceland shelf and that farther south, both in rift propagation pattern and free air gravity lineations. Furthermore, we identify two small offset features coined ponsu-transforms, from which rift propagation is both initiated and stopped. The pattern of the V-shaped ridges on the Reykjanes Ridge is not symmetric about the Reykjanes Ridge and the V-shaped ridges are not linear continuous features. Our rift propagation model produces excellent fits to magnetic data and provides a self-consistent model for the evolution of the Reykjanes Ridge during the past 15 Ma.

Description: IODP Expedition 340 successfully drilled a series of sites offshore Montserrat, Martinique and Dominica in the Lesser Antilles from March to April 2012. These are among the few drill sites gathered around volcanic islands, and the first scientific drilling of large and likely tsunamigenic volcanic island-arc landslide deposits. These cores provide evidence and tests of previous hypotheses for the composition and origin of those deposits. Sites U1394, U1399, and U1400 that penetrated landslide deposits recovered exclusively seafloor-sediment, comprising mainly turbidites and hemipelagic deposits, and lacked debris avalanche deposits. This supports the concepts that i/ volcanic debris avalanches tend to stop at the slope break, and ii/ widespread and voluminous failures of pre-existing low-gradient seafloor sediment can be triggered by initial emplacement of material from the volcano. Offshore Martinique (U1399 and 1400), the landslide deposits comprised blocks of parallel strata that were tilted or micro-faulted, sometimes separated by intervals of homogenized sediment (intense shearing), while Site U1394 offshore Montserrat penetrated a flat-lying block of intact strata. The most likely mechanism for generating these large-scale seafloor-sediment failures appears to be propagation of a decollement from proximal areas loaded and incised by a volcanic debris avalanche. These results have implications for the magnitude of tsunami generation. Under some conditions, volcanic island landslide deposits comprised of mainly seafloor sediment will tend to form smaller magnitude tsunamis than equivalent volumes of subaerial block-rich mass flows rapidly entering water. Expedition 340 also successfully drilled sites to access the undisturbed record of eruption fallout layers intercalated with marine sediment which provide an outstanding high-resolution dataset to analyze eruption and landslides cycles, improve understanding of magmatic evolution as well as offshore sedimentation processes. This article is protected by copyright. All rights reserved.

Description: The study of air-sea CO 2 fluxes ( F CO 2 ) in the coastal region is needed to better understand the processes which influence the direction and magnitude of F CO 2 and to constrain the global carbon budget. We implemented a one-year (January through December, 2009) paired study to measure F CO 2 in the intertidal zone (the coastline to 1.6 km offshore) and the near-shore (~3 km offshore) off the north-western coast of Baja California (Mexico); a region influenced by year-round upwelling. F CO 2 was determined in the intertidal zone via eddy covariance; while in the near-shore using mooring buoy sensors then calculated with the bulk method. The near-shore region was a weak annual net source of CO 2 to the atmosphere (0.043 mol CO 2 m -2 y -1 ); where 91% of the outgassed F CO 2 was contributed during the upwelling season. SST and Δ p CO 2 (from upwelling) showed the strongest relationship with F CO 2 in the near-shore suggesting the importance of meso-scale processes (upwelling). F CO 2 in the intertidal zone were up to four orders of magnitude higher than F CO 2 in the near-shore. Wind speed showed the strongest relationship with F CO 2 in the intertidal zone suggesting the relevance of micro-scale processes. Results show that there are substantial spatial and temporal differences in F CO 2 between the near-shore and intertidal zone; likely a result of spatial differences. We suggest that detailed spatial and temporal measurements are needed across the coastal oceans and continental margins to better understand the mechanisms which control F CO 2 , as well as reduce uncertainties and constrain regional and global ocean carbon balances.

Description: Recently acquired high-resolution multichannel seismic profiles together with bathymetric and sub-bottom profiler data from the external part of the Gulf of Cadiz (Iberia-Africa plate boundary) reveal active deformation involving old (Mesozoic) oceanic lithosphere. This area is located 180 km offshore the SW Iberian Peninsula and embraces the prominent NE-SW trending Coral Patch Ridge, and part of the surronding deep Horseshoe and Seine abyssal plains. E-W trending dextral strike-slip faults showing surface deformation of flower-like structures predominate in the Horseshoe Abyssal Plain, whereas NE-SW trending compressive structures prevail in the Coral Patch Ridge and Seine Hills. Although the Coral Patch Ridge region is characterized by subdued seismic activity, the area is not free from seismic hazard. Most of the newly mapped faults correspond to active blind thrusts and strike-slip faults that are able to generate large magnitude earthquakes (Mw 7.2 to 8.4). This may represent a significant earthquake and tsunami hazard that has been overlooked so far.