Prepared by Brendan Carter

A map of our section, with the yellow arrow highlighting what we’ve done this week.

We have had another productive week.nWe crossed the Equator and started our journey across the Southern Hemisphere, completing 33 stations and the rest of our high-resolution Equatorial work in the process. We’ve had very few delays this week, and no significant ones. nWe are making good time, and are on track to make it far enough South to have a short and efficient transit to our end-of-leg-1 port call on Easter Island.

We are now on the diagonal that takes us from the 110°W meridian to the 103°W meridian. As I write this we are on one of the pair of shallower (~3000 m) stations directly over the East Pacific Rise. The East Pacific Rise is a mid-ocean spreading ridge, a massive underwater feature of the Earth’s crust that has fascinating physical and geochemical interactions with the ocean above it (it can be seen as a lighter blue band in the map above).  Some of our analysts are specifically aiming to measure the signatures of these interactions.  The P18 line angles NW/SE between 5° and 10° S to capture this feature while still resolving the deeper ocean basin between the East Pacific Rise and the Peru-Chile Trench. There will be a long set of deeper stations as we come down off the Rise and continue South into that basin in the remaining two weeks of leg 1.

One exciting bit of news is that we will have a Chilean observer joining us in Easter Island. The observer is trained as a marine biologist at the Universidad de Valparaiso, and we look forward to having her input and company.

Equatorial Currents

SADCP measurements of the eastward current velocities (top) between 8° and 18°N and (bottom) around the equator. The white areas are where there weren’t strong enough echoes to make a reliable measurement. The alternating deep and shallow readings on the top plot hint at what time of day it was when the boat was making the measurements. The deeper measurements are from the day when many small organisms go down to the darker depths to hide from predators. At night, the critters return to the surface to feed on even smaller organisms, so the SADCP doesn’t have anything good to bounce sound waves off of at depth. Credit for this data belongs primarily to Jules Hummon who leads the SADCP data collection effort.

As mentioned in the last update, the narrow band around the Equator has strong and unique currents that are possible due to the disappearance of the Coriolis force at the boundary between the Northern and Southern Hemispheres. Our shipboard Acoustic Doppler Current Profiler (or SADCP) can measure these currents directly by pinging the ocean with sound waves and measuring the frequency (pitch) of the echoes. The echoes bounce off of things caught up in the currents (mostly small plankton). Two plots are provided for comparison, both showing the eastward current versus depth across a range of latitudes. The top one is from further north and shows a typical current structure. The second plot shows the much stronger and more confused current structure expected across the Equator (0°N on the figure).

Staying on station while also keeping the CTD rosette package more-or-less directly below the boat as the package sank through the strong and variable Equatorial currents provided an extra challenge for the Brown’s crew. However, they were successful, and we completed all of our Equatorial work without any significant problems. The Brown’s crew should also be commended on having recently repaired the ship’s POSMV system in January of 2016. This system provides accurate position, heading, and velocity information that the SADCP needs to make accurate measurements of the currents.

Shipboard measurement differences

Preliminary measured distributions of total dissolved inorganic carbon (DIC: left panel), and thenincreases (red) in DIC relative to concentrations measured during the previous P18 occupation in 2007/2008 (right panel). The concentration changes shown are relative to the measured DIC in the earlier occupation interpolated onto the same neutral density surface that the water was on when we measured it on this occupation. These data are brought to you through hard work from analysts Bob Castle and Remy Okazaki

We’ve begun plotting many of the properties we’re measuring on board and mapping the differences between our measurements and the measurements from decades past. This is primarily a quality control exercise. Very large differences between the decades can be a sign of problems with the analysis or data entry, especially when only a single measurement shows the large difference. Fortunately, the data appear to be quite clean so far.

Of course the other reason to make difference plots is that we are all very eager to see how the seawater properties have changed since the last time scientists measured the P18 section (even though we recognize the data we are producing are still too preliminary to do solid research with). As an example, the plot above shows the increase in total dissolved inorganic carbon (DIC) over the last decade on surfaces of constant density mapped back onto a depth section. We expect to see increases in DIC as the ocean continues to absorb the CO2 that humans are emitting into the atmosphere. This plot shows the expected increases have continued over this last decade, especially near the ocean surface. However, finalized fully-calibrated data and a more thorough analysis will be required to accurately quantify how much of this increase is attributable to human CO2 emissions.

Life at sea

Pictures of Mari (our dissolved organic matter and radiocarbon sampler) enjoying the sunset (top), and Christian (who is currently involved in too many measurements to easily list) making sure none of our hard-earned seawater is going to waste (bottom). Brett Walker kindly supplied both pictures.

We’ve been working long days without weekends or vacations for several weeks, and many of us spent weeks more in port in San Diego.I won’t presume to speak for everyone, but I know I am missing the people, pets, and places that make up my life on land. Fortunately, we have a brilliant, joyful, and hardworking team of scientists and crew aboard making the long days and nights go quickly and with no small amount of fun.nIt also helps that the persistent beauty of the sea, the sky, the sun, and the stars are always just beyond the nearest bulkhead.nFor a more about how (and why) we are spending our days at sea, consider checking out the P18 blog.

Summary

It has been a good week for most aboard, and the science and crew team should feel proud of hard work they did through the Equator crossing. Nevertheless, we are steeling ourselves for another two weeks of focused work and trying not to daydream too much about the island that awaits us at the end of this first leg of P18.