Every day at sea usually provides at least one surprise.The Orca Basin has provided us with quite a few big surprises, the most striking of which was the discovery of red and pink (!)sediments in the middle of the basin. In the Northern and Southern mini-basins, we collected cores of black, extremely sulfidic mud. When we sent the multiple-corer over the side in the central Orca Basin, we expected to retrieve something similar but much to our surprise, the mud recovered from the more shallow (2000 vs. 2200m) central basin was red (see image gallery below for more photos). Red and pink deep sea sediment…where in the world does that come from?
We don’t know so ALVIN dive 4650 aimed to explore this area in detail. At this point, I have to give long overdue credit to the ALVIN group, particularly expedition leader Bruce Stickrott. Without them, we would not be able to accomplish the amazing things we do! As we headed towards the bottom, we passed through several different interfaces with thick brine fog; each layer was distinct, the most shallow had swimming amphipods, the next two deeper, anoxic layers were simply microbial wonderlands. When we finally reached the seafloor, the surface was orange/brown, typical of sediments coated by iron-oxyhydroxides (we think...lab experiments are required to clarify this) but with pink sediment below.
When we collected sediment cores at this location, we saw that the thin surficial brown layer covered, yes, the hot pink layer sampled in the multiple core samples. We drove up the ridge, which was strangely enough, terraced, and at the edges of these terraces, there appeared to be brine flows: dark, reduced patches where if one looked closely, you could see the shimmering produced when fluids of different densities mix. This is one of the many reasons I love ALVIN: there is no substitute for seeing these things with your own eyes. It’s so different from looking at the video feed from an ROV. There is no replacement for “being there” and seeing things live, even if it’s cramped, cold and you’re looking through a tiny viewport.
We collected sediment and brine fluid samples as we traversed up the ridge and were amazed the entire time by the pervasive pink sediment underlying the orange/brown surface layer. The pink sediments had the consistency of freshly mixed, but not yet firmed, jello, and were quite a challenge to sample. But our pilot Bruce is an ace at coring and we got some great cores. Once we moved further up the slope, we entered the sponge garden area. Here, sponges are abundant on the seafloor and the sediment is still pink. We saw at least five or six different species of glass sponges and each was spectacularly beautiful. I’ve posted more images of sponges in the gallery below. Our hypothesis is that the pink color is due to some pigment that the dominant microorganism carries. It will be exciting to tease apart this pink microbial mystery.