Finding Opportunity in Misfortune

21 September 2013

9^o 30' N
28^o 15' W

Hurricane Humberto certainly sent us running, but aside from rough seas, we were able to avoid any significant weather by adjusting our course. We were able to return to the 29^o W line and work our way north behind the storm, to make up our missed stations before returning to our southward track.

Unfortunately, escaping Humberto was not to be the end of our worries. Since Madeira, we have suffered ongoing problems with the cable that deploys the ship's instruments and sampling bottles. Since we're spooling out almost 6,000m (~3.5 miles) of cable to sample the bottom waters of the ocean, little offsets add up quickly. Over the last two weeks of operations, the cable often became kinked and uneven as it spooled onto the winch.

To remedy these delays, the scientists and crew decided to move the instrumentation package to an alternate winch with newer cable (1.5 years old). Everything seemed to be performing excellently, until a weak spot in the cable was discovered, with almost 3,000 m of cable spooled out. Although the crew was quick to spot the problem and respond, the cable broke suddenly, and the instrumentation fell to the ocean floor.

(To see high-resolution pictures of a CTD-rosette, visit the Hiawaii Ocean Time series website here:  www.soest.hawaii.edu/HOT_WOCE/ph_new_CTD.html).

A back-up system was quickly outfitted with spare instruments and bottles, but the problem with the cable remained. Several fixes were attempted over the next two days, and finally a solution was discovered. The top wraps of cable were discarded, and the remaining cable was found to wind neatly onto the winch, without kinks, catches, or gaps. As soon as possible, the winch and back-up system were tested thoroughly, and we resumed our sampling schedule, albeit with a few modifications to the cruise plan. (To track the Ronald H. Brown, follow this link: shiptracker.noaa.gov/shiptracker.html).

While the scientists and crew worked diligently to repair the primary sampling system, the trace metal operations were allowed to continue. The ship stayed at 15^o N 29^o W for almost three days, to offer the safest working environment for the repair work to proceed. It just so happened that for these entire three days, we were located right in a significant Saharan dust event. While these events are most common during the fall season, they are still unpredictable. To be able to repeatedly sample this extended event was an unexpected opportunity that we took full advantage of.

As seen in the figure below, the high loads of dust make our paper filters look almost like an emery board. These high aerosol dust loads translate into higher concentrations of dissolved and particle iron in the ocean. And just as expected, we have also seen an increase in the colonies of marine cyanobacteria called Trichodesmium, that use iron to take atmospheric nitrogen and convert it to amino acids and proteins.

An unused filter on the left contrasted with a dust-laden filter on the right.
These high-dust conditions lasted for 3-4 days, producing almost 50 subsamples to analyze and experiment with.

For further reading;  http://www.whoi.edu/science/B/people/ewebb/Tricho.html