Search

The complex topography of submarine canyons may result in different composition of benthic foraminifera assemblages. To understand how trophic, hydrological and sedimentological conditions in submarine canyons can influence the distribution of benthic foraminifera, and to use this information to corroborate paleoenvironmental interpretations for the Holocene, we investigated recent benthic foraminiferal assemblages (total fauna >63 μm) and sedimentological data in two canyons (CANWN and CAND) in the Espírito Santo Basin (ESB) between 18°20′ and 21°20′ S. Surface sediment samples (0–2 cm) were collected inside the canyons (150 to 1300 m water depth) and in shelf-slope adjacent transects (50 to 1300 m water depth). The density, taxonomic diversity, and assemblage composition of benthic foraminifera change with depth and location. The distinct ecological preferences of the most abundant taxa allowed us to recognize five benthic foraminiferal groups. Three groups (V, III, and I) are present in different bathymetric sectors; Group V: outer shelf (50 m), Group III: upper, and middle – lower slope (150 to 400 m), and Group I: middle – lower slope (1000 to 1300 m). Groups II and IV show no characteristic bathymetric distribution and are present only in CAND and in CANWN, respectively. Group V consists of Hanzawaia boueana , Peneroplis planatus , and Quinqueloculina lamarckiana; Group III is dominated by Globocassidulina rossensis and Trifarina spp.; Group I consists of Globocassidulina crassa , Bolivina lowmani , Gavelinopsis versiformis , Alabaminella weddellensis , and Epistominella exigua. The main species in Group II (CAND, 150, 1000 to 1300 m) are Trifarina angulosa , Globocassidulina subglobosa , and Discorbis vilardeboanus. Group IV (middle – lower CANWN, 1000 to 1300 m), consists mainly of agglutinated species Glomospira charoides , Rhabdammina abyssorum , and Psammosphaera fusca. Our data suggest that the quantity (and quality) of food supply, hydrodynamic conditions and sediment properties are the main drivers controlling the bathymetric distribution of benthic foraminiferal assemblages in both canyons. The middle – lower CANNW revealed unstable trophic conditions, related to terrigenous sediment input due to turbidity currents. In CAND, the foraminiferal assemblages ecology indicated sufficient organic matter supply that favors species establishment and diversity, indicating a more productive and less unstable environment than in CANWN. • Environmental factors related to submarine canyons influence the benthic foraminiferal distribution. • CANWN and CAND assemblages are different from the open slope. • The agglutinated assemblage in the middle-lower CANWN may reflects a mass-transport movement. • Benthic foraminiferal species abundance indicated enough organic matter trapped within CAND. [ABSTRACT FROM AUTHOR]

The 2016 Mw 7.8 Kaikōura Earthquake triggered simultaneous turbidity currents down ten submarine canyons along a 200 km stretch of the continental slope, east of New Zealand. They discharged into the abyssal Hikurangi Channel which flows >1500 km northwards along the trench floor as part of the Hikurangi Subduction Margin. One hundred and thirty-six foraminiferal samples from the 2016 turbidites and pre-turbidite sediment from 25 canyon/channel cores, collected during surveys 3 days, 2.5 and 8 months after the event, were used to provide insights into the complexities of these submarine gravity flows. Transport in the turbidity currents produced no significant additional breakage of foraminiferal tests, even when transported 650 km down channel. We conclude that highly variable planktic foraminiferal fragmentation index (FI) values indicate that test breakage in turbidite samples is primarily inherited from dissolution on the seafloor at their source area, which is often anomalously high because of lowered sediment pH as a result of the breakdown of the high organic carbon flux. All turbidite and pre-turbidite faunas have strong signatures of downslope displacement (presence of shallow-water benthics, size-sorted test distributions, low percentage of planktics). This is not surprising as all cores were taken in the canyon or channel floor pathways to collect turbidite sequences, with turbidity flows estimated to have a recurrence time of ∼150 yrs. Pre-turbidite faunas can be distinguished from the 2016 turbidites' faunas by having greater percentage of planktics and lower ratios of shallow to deep benthic tests (S/D Index). 210Pb profiles show that the laminated pre-turbidite sediment accumulated slowly and the foraminiferal content indicates it is composed of reworked earlier turbidite sediment with the addition of contemporaneous deep-water benthic tests and the rain of planktic tests mixed in. There are no consistent trends in any foraminiferal faunal parameters with distance displaced in the turbidity current (10–650 km). Rather, intra-turbidite (10–75 cm thick) faunal variability in a single core is often considerable and is attributed to erosion and entrainment of deep-seafloor tests by the passage of turbulent turbidity current heads, variable source canyon depths, different source canyon provenances, and test-size sorting during transport and deposition resulting in vertical and longitudinal fractionation of the smallest tests and finest sediment. This study suggests that a single foraminiferal fauna is often, but not always, insufficient to characterise a turbidite or for use in correlation with turbidites in other cores or exposures. • Foraminiferal tests showed little breakage during 650 km of turbidite transport. • Intra-turbidite foraminiferal variability is often considerable. • Turbidity currents sometimes erode and entrain deep-seafloor foraminifera. • Foraminiferal faunas can identify different turbidite provenance areas. [ABSTRACT FROM AUTHOR]

In 2014, the International Ocean Discovery Program (IODP) Expedition (Exp.) 349 discovered carbonate layers at sites located in the South China Sea (SCS) central basin. Preliminary results from shipboard investigation showed that most of these carbonate layers are calcareous nannofossil ooze and interpreted them as turbidites. However, more detailed studies are required to determine their depositional mechanisms and paleoenvironmental implications. In this study, we have analyzed CaCO 3 content, calcareous nannofossil abundance, assemblage composition and preservation in carbonate and clay samples from the upper Miocene sections of Site U1433 in the SCS central basin. We confirm that these carbonate layers are of turbidity current genesis but are not typical nannofossil ooze, while the clays represent regular in situ deep sea sediments. The study interval spanned approximately from 10 to 5 million years before present (Ma). Calcareous nannofossil abundance and assemblage are used to reconstruct the evolution of the upper seawater structure in the SCS central basin. Our results indicate that the seawater shifted from strongly stratified to well-mixed due to enhanced monsoon. This shift was accompanied by an increase in pelagic productivity at ~8 Ma. This environmental change is critical for component variations of the carbonate turbidites since we suggest that these turbidites mainly sourced from periplatform ooze in the southern SCS composed by of mixed neritic and pelagic particles. The neritic and pelagic CaCO 3 production have inversed responses to the change of seawater environment in the SCS and thus cause the component variations in the carbonate turbidites of Site U1433. • Upper Miocene carbonate layers in the South China Sea central basin comprise varying amounts of calcareous nannofossils. • The carbonate layers and pelagic clay layers of Site U1433 represent turbidity-dominated and in situ productivity-dominated depositions respectively. • A prominent calcareous nannofossil change at about 8 Ma was related to the enhanced East Asian Monsoon and the increas in the Mekong River discharge. [ABSTRACT FROM AUTHOR]