Why are denser manganese nodules atop the seabed?

Sea floor sediment precipitates at a much faster rate than manganese nodule accretion, so why do they stay atop the less dense sea floor?

I remember in a marine science class decades ago there were some ideas, but I’ve been out of the loop on it since then.

Not really. While a nodule radius might take several million years to grow by 1 centimeter, deep sea sediments could also take that long to accumulate. And remember that nodule formation is a surface activity. Metal hydroxides form concretions around a core. That can’t happen underneath the soft sea floor. But a lot of nodules are partly buried in sediments. No doubt a lot of them have been completely covered and lost from view.

Not really - even the slowestpelagic sedimentation rates are something like 2 orders of magnitude faster than nodule accretion rates.

This is also inaccurate. Nodule growth is a balance of hydrogenous growth (characterized by vernadite) and diagenetic growth from sediment-penetrative seawater (characterized by birnessite) with the relative contribution of each process varying in different localities.

This is all true.

There are two factors that some consider may account for the location of nodules - one is that the largest accumulations occur in localities where bottom currents sweep the lighter clays and oozes away and leave the heavier nodules, another is that bioturbation may be responsiblefor keeping them at the surface (at least for certain size ranges of nodule).

Are the modules more-or-less mythis? I understand that the only major mining excerscise was just a cover for a secret submarine recovery operation.

Nope. Uneconomical to mine =/= mythical.

I’ve seen them with my own eyes, come straight off a marine research vessel. Done petroscopy on one, too.

But then, the given rate of 0.1-.05 cm/1000 years is for areas wherein there are noticeable deposits of pelagic clay sediments. Not all of the ocean floor has it. In some parts, deposition is very slow. As one geology professor of mine pointed out, less than two feet of deep sea pelagic clay can already span an entire era in geologic time. But we know that solidified ooze and pelagic clay contain some manganese nodules that have been buried, along with things like whale bone and teeth, so yes, burial of the nodules do happen.

Well yeah - only ~38% of the ocean floor. Most of the rest is covered in the* even faster-deposited* calcareous ( ~48%, 0.3–5 cm/1000 yr) and siliceous (~15%, 0.2–1 cm/1000 yr) oozes. Very little of the ocean floor is bare rock or other sediment - I’d guess only the active ridges and the continental shelf, really.

I’d certainly consider .05cm/Ma to be slow. But it’s still orders of magnitude faster than nodule growth - hence the need for explanations as to why nodules seem to occur out of expected proportion at the water-sediment interface

2 feet for* at minimum* 66 million years (the shortest geological era)? I highly doubt it, given the Atlantic Cretaceous/Paleogene red clays have layers >100m in thickness. The parts of the ocean with clay that thin is also the parts next to the ridges, below the CCD, which are the newest parts and certainly aren’t spanning an era.

Unless he was using “era” in some non-technical sense, in which case, sure, I guess, but that doesn’t mean anything.

That’s what I agreed about, and my last cite also showed. In that particular locality, it seems this happens when the nodule gets too large for bioturbation.

Looks like the case for biotubation has gotten stronger.


Good job!