Wednesday, 25 July 2012

Day 20 - Like a Rolling Stone

While others are acquiring spectacular images of the seafloor with Autosub, I’ve been amusing myself by... um… collecting stones. We find these embedded on the surfaces of megacores. Most of the stones originate from the higher points on the abyssal seafloor (i.e. the shallower sites), probably because these are areas where they are exposed by sediment winnowing.

Surface of stone with delicate network of Telammina; note the tiny
chambers strung out along the very narrow tubes that make up the network.
This genus is common on hard substrates at many sites in the deep ocean
A few are ‘clinker’ – lumps of burnt coal discarded by steam ships. Clinker is a very common human contaminant of the ocean floor, which is surprising since steam ships only operated for a period of about 100 years. However, most of the stones are bits of rock, almost certainly dropped by melting icebergs during the last glacial period. They often have a black coating, probably of iron-manganese oxide.

Despite initial appearances, deep-sea stones can be quite interesting. The ones that are exposed at the surface provide a hard substrate on which different creatures can settle, and are often teeming with life.

A stone with a brachiopod (Pelagodiscus) attached to the surface.
Also visible are two mat-like formations, one dark grey and
the other whitish in colour
During this cruise we found animals such as brachiopods and bryozoans on the stones. However, by far the most abundant organisms were foraminifera, single-celled protozoans that Laetitia has already written about. (Day 16)

Most foraminifera – forams – live in the soft sediment but others fix themselves to hard substrates. We found a wide variety of these attached forams on the stones.

This solid dome shows some internal structure suggesting the presence
of chambers within the dome. This suggests that it is a foraminiferan belonging to the 
Komokiacea.  This superfamily is extremely common in the deep sea, although
only a few species live attached to hard substrates

Some were simple mud domes, others included extensive networks of fine tubules and flat mat-like formations. But the most common was a delicate net of minute tubules with tiny chambers positioned along them. Unlike most of the encrusting forams, this one has a name – Telemmina.

The stones provide sanctuaries on the seafloor for foraminifera and animals that could not survive in the sediment. In the Pacific Ocean, manganese nodules are often densely covered with similar organisms. Nodules are much less common in the Atlantic Ocean, so it’s fascinating to see the same kinds of foraminifera settling on stones that were transported from distant continents by icebergs thousands of years ago.

An extensive network of fine tubules covers a large area of
this stone, while on the left-hand side is a mat-like crust. Both are
assumed to be foraminifera.

Andrew Gooday