CTD Tow-yo

Monday August 17, 2015
By Rachel Teasdale

Weather: Occasional light showers with overcast skies, but the winds have come up to 20-25 knots, and are forecast to get higher tomorrow.

What’s happening today? This morning ROV Jason was recovered early because the winds and seas were building. In the afternoon AUV Sentry was recovered and tonight we will start a 6-hour CTD Tow-Yo.

Axial Seamount 2015 Expedition video about the suite of instruments and diversity of science associated with the CTD instrument package. Video by Jesse Crowell in association with Saskia Madlener at 77th Parallel Productions. Music by James Andrew Menking

CTD during launch on 14 August 2015 to measure background conditions from which water from hydrothermal plumes will be compared.

A CTD is a basic oceanographic tool and serves multiple purposes in studying the physical and biological conditions of ocean water. A collection of instruments are housed on a CTD to measure Conductivity (to calculate water salinity), Temperature, and Depth, in addition to other variables. There is also a series of 24 bottles in which seawater can be collected on command for later chemical analysis. The vertically oriented bottles are arranged in a circle, making a “rosette” of bottles (see image of CTD at right).

The CTD rosette is lowered from the ship’s deck usually to just above the seafloor and then raised back up again. The CTD sensors measure continuously as the CTD ascends or descends, providing real-time information about the properties of the ocean. On this cruise, Nathan Buck is using the CTD data to identify regions of hydrothermal plumes (often rich in chemicals and microbial life) that are emitted from vents on the ocean floor. When the sensors indicate the CTD is in a hydrothermal plume, then individual water samples can be collected at various depths in the rosette of bottles on the CTD.

Cartoon of tow-yo pattern for CTD measurements, from www.venturedeepocean.org/tools/tow_yo.php

On August 14th CTD data and water samples were collected north of Axial Seamount as a reference of “background” conditions outside the influence of the volcano. These data will then be compared with samples closer to the hydrothermal vents, the caldera, and the new lava flows. Vertical CTD casts like that on August 14th collect water samples as the CTD rosette is lowered and raised vertically, without any lateral movement of the ship. Tonight, the CTD will be raised and lowered as it is towed slowly behind the ship, making a zig-zag or yo-yo type pattern. The combination of the towing and yo-yo-ing is referred to as a “tow-yo” (see cartoon of tow-yo at left).

Tonight’s tow-yo will search for hydrothermal plumes associated with 2015 lava flows of Axial’s North Rift Zone. The CTD will collect samples across a zone 8 km (5 mi) long and 200 m (640 ft.) tall, from 1800 to 1600 m (5760- 5120 ft.) depth. In this way, the CTD will define the location and intensity of the plume and will provide information about its chemical composition and what microbes may be present in it. This will tell us what is coming out of the hydrothermal vents on the new lava flows and what impact they are having on the overlying ocean.

CTD Rosette prior to launch 18 August 2015. Note vertical Niskin bottles that will collect water samples during the CTD tow-yo.

Once on board, each water sample (photo right) is divided among the different research groups, who will study the gases, ions, and microbiology of the plume. For example, some of the water samples will be analyzed by Emily Reddington on this cruise to determine which microbial organisms (microbes) are present in the plumes, which will involve DNA analyses to identify specific organisms. Some samples will be analyzed to characterize the activity of microorganisms (e.g. based on CO2 uptake) and others will be prepared for later analyses of viruses that may be present. Some of this can be completed on the ship, but many samples are preserved and stored for later analysis at laboratories back on shore in the US and Canada. These studies help scientists identify the diversity of organisms that live in the hydrothermal plumes and to recognize the environmental conditions in which those organisms thrive. The CTD work on this cruise will show what changes have occurred in the hydrothermal plumes at Axial as a result of the 2015 eruption.

More information about CTDs is available at:
http://www.pmel.noaa.gov/eoi/PlumeStudies/WhatIsACTD/CTDMethods.html