『Abstract
The chemical composition of ferromanganese nodules from the three
nodule-bearing MANOP sites in the Pacific can be accounted for
in a qualitative way by variable contributions of distinct accretionary
processes. These accretionary modes are: (1) hydrogenous, i.e.,
direct precipitation or accumulation of colloidal metal oxides
in seawater; (2) oxic diagenesis which refers to a variety of
ferromanganese accretion processes occurring in oxic sediments;
and (3) suboxic diagenesis which results from reduction of Mn+4
by oxidation of organic matter in the sediments. Geochemical evidence
suggests processes (1) and (2) occur at all three MANOP nodule-bearing
sites, and process (3) occurs only at the hemipelagic site , H,
which underlies the relatively productive waters of the eastern
tropicl Pacific.
A normative model quantitatively accounts for the variability
observed in nearly all elements. Zn and Na, however, are not well
explained by the three end-member model, and we suggest that an
additional accretionary process results in greater variability
in the abundances of these elements. Variable contributions from
the three accretionary processes result in distinct top-bottom
compositional differences at the three sites. Nodule tops from
H are enriched in Ni, Cu, and Zn, instead of the more typical
enrichments of these elements in nodule bottoms. In addition,
elemental correlations typical of most pelagic nodules are reversed
at site H.
The three accretionary processes result in distinct mineralogies.
Hydrogenous precipitation produces δMnO2.
Oxic diagenesis, however, produces Cu-Ni-rich todorokite, and
suboxic diagenesis results in an unstable todorokite which transforms
to a 7 Å phase (“birnessite”) upon dehydration. The presence of
Cu and Ni as charge-balancing cations influence the stability
of the todorokite structure. In the bottoms of H nodules, which
accrete dominantly by suboxic diagenesis, Na+ and possibly
Mn+2 provide much of the charge balance for the todorokite
structure.
Limited growth rate data for H nodules suggest suboxic accretion
is the fastest of the three processes, with rates at least 200
mm/106 yr. Oxic accretion is probably 10 times slower
and hydrogenous 100 times slower. Since these rates predict more
suboxic component in bulk nodules than is calculated by the normative
analysis, we propose that suboxic accretion is a non-steady-state
process. Variations in surface water productivity cause pulses
of particulate flux to the sea floor which result in transient
Mn reduction in the surface sediments and reprecipitation on nodule
surfaces.』
Introduction
Analytical procedures
Compositional variations
General patterns
Top-bottom contrasts in composition
Mineralogical variability
Variations in proportion of charge-balancing cations
Normative nodule analysis
Procedures
Results of model
Perturbations on the three end member model
Conclusions of the normative modeling
Distributions of other minor elements
Accretion rate considerations
Rate and compositional summary of three accretionary processes
Accretion rate - normative composition dilemma
Summary of nodule accretionary processes
References