Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
• Data Files
• Related Publications
• Parameters
• Instruments
• Deployments
• Project Information
• Funding

Benthic fluxes of solutes where determined with three in-situ benthic flux chambers (BFC) according to Treude et al. (2009). The BFCs consisted of a frame equipped with a cylindrical polycarbonate chamber (inner diameter = 19 centimeters) with its lower portion sticking out of the frame. The upper side of the chamber was closed by a lid containing a stirrer (Type K/MT 11, K.U.M., Kiel, Germany), oxygen optodes (Type 4330, Aanderaa Data Instruments, Bergen Norway and Hydroflash, Contros/Kongsberg Maritime, Kongsberg, Norway), a conductivity sensor (type 5860, Aanderaa Data Instruments), and a valve for water to escape when the chamber is inserted into the sediments.

Prior to deployment by the ROV Jason, the chambers were held upside down by the ROV manipulating arms within approximately 10 meters of the seafloor and moved back and forth to make sure water from shallower depths that may have been trapped was replaced by bottom water. Chamber incubations lasted between 240 and 390 minutes. Each BFC was outfitted with a custom-built syringe sampler containing seven 50-milliliter (mL) syringes that were connected by tubes to sampling ports in the upper portion of the wall of the chambers: one injection syringe and six sampling syringes that were fired at approximately 60-minute increments, depending on the total duration of the incubation. The injection syringe contained deionized water that was purged with helium to remove oxygen and the volume of the overlaying water (typically 2.5 to 4 liters depending on sediment characteristics and chamber handling by ROV) was determined based on the observed reduction in conductivity after sensor readings stabilized (i.e., full mixing was achieved) 10-30 minutes after injection (Kononets et al., 2021). Samples obtained from the overlaying water of the BFC were examined for NH4+, PO43-, Fe2+ (according to Grasshoff et al., 1999), and NO3- (according to García-Robledo et al., 2014). Benthic fluxes of the solutes were calculated as follows:

J=Δc/Δt*V/A

Where J is the diffusive flux in millimoles per square meter per day (mmol/m²/d), Δc is the concentration change in millimoles per cubic meter (mmol/m³), Δt is the time interval in days (d), V is the overlying water volume in cubic meters (m³), and A is the surface area of the sediment covered by the benthic flux chamber in square meters (m²). One chamber per site contained 200 micromoles 5N-NO3- in the injection syringe for in-situ nitrogen cycling experiments. The injection of 15N-NO3- roughly doubled the concentration of NO3- in the overlying chamber water compared to the natural concentration. The effect of the nitrate addition on conductivity and, hence, volume determination was negligible (<1%).

BCO-DMO Processing:
- renamed fields to comply with BCO-DMO naming conventions;
- converted the date-time fields to ISO 8601 format;
- removed "-" and "no entry" as missing data values (missing data/no data values are blank/empty in the final csv file).

NSF Award Abstract:
This study focuses on chemical processes that occur in oxygen-limited waters along the world's continental margins. These processes are influenced by the activities of microbes and control the fate of key elements that are deposited to sediments in these areas including carbon, nitrogen and sulfur. As a result, they are key to the health and function of the ocean. The intellectual merit of this research is to study the coupled chemical and microbial processes that occur in these environments by combining robotic technology with experiments that will be conducted at the ocean floor and in the shipboard laboratory. The broader impacts of this project will provide at-sea training and educational opportunities to undergraduate and graduate students and the results will be broadly distributed to stakeholders and interested parties. Results from this research promise to identify and quantify rates for key processes that couple carbon, nitrogen and sulfur in marine environments adjacent to the continents. The project addresses an important aspect of environmental change in the ocean (i.e., decreased oxygen due to warming and nutrient enrichment) and its influence on chemical and biological cycles and ocean ecosystems.

The dynamics of oxygen minimum zones along continental margins, and their potential for future expansion, are important because of their intersection with global biogeochemical cycles and because of their far-reaching impacts on ocean ecosystems. However, the impacts of transient deoxygenation on biogeochemical cycles of carbon, nitrogen and sulfur at the sea floor are not well established and are the focus of this study. This study will test the overarching hypothesis that deoxygenation triggers a positive feedback loop between bacterial mats at the sea floor that consume hydrogen sulfide, a sulfur species that can be toxic to higher organisms, and an underlying community of bacteria that produce hydrogen sulfide. By this hypothesis, the establishment of sea floor mats, which depend on inorganic nitrogen sources to run their sulfur metabolism, accelerates nitrogen cycling in the uppermost sediment horizon following deoxygenation. The accelerated nitrogen cycling allows for upward expansion of the sulfide-producing bacteria, which in-turn provide a shallow source of sulfide as substrate to further support nitrogen cycling in the sea floor mat. The results of this study will enable understanding of the relationship between oxygen dynamics in the water column and the biogeochemical processes at the sea floor that link the transformations of carbon, nitrogen and sulfur. The results of this study promise to define the environmental conditions under which the sulfur and nitrogen cycles are coupled and subject to strong positive feedbacks at the seafloor, as well as the conditions under which they are decoupled. This study provides training in research and innovative analytical and experimental techniques to four graduate students and several undergraduates. Undergraduates will be engaged in research at two institutions, one of which has recently been designated as a Hispanic serving institution. Approximately 10 undergraduate students (20 in total) will participate in each of the two proposed oceanographic expeditions, through an established course entitled: Field Studies in Marine Biogeochemistry. This course provides an opportunity for students to develop an independent research project in advance of the expedition, to participate on the expedition, and to conduct research projects while at sea.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.