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doi:10.1130/ G35636Y.1
Seawater chemistry driven by supercontinent assembly, breakup and dispersal
R.D. Müller1, A. Dutkiewicz1, M. Seton1, and C. Gaina2
1
EarthByte Group, School of Geosciences, The University of Sydney, NSW
2006, Australia
2
Centre for Earth Evolution and Dynamics, University of Oslo, Oslo 0316,
Norway
We thank Norton and Lawver (2014) for their comment, which
provides us with an opportunity to address the issue of geological
uncertainty. We agree that our 200 Ma reconstruction needs to be
adjusted to remove convergence between the Izanagi and Farallon
plates. We moved the Izanagi-Farallon stage pole westward and
adopted other changes in the circum-Arctic as proposed by Shephard et
al. (2013) to produce a new 200 Ma age grid (Fig 1A), resulting in a 1–
65 m.y. old ocean floor area of ~192 × 106 km2. We are unsure how
Norton and Lawver derived their estimate of 219 × 106 km2 and it is
difficult to understand why they show our 200 Ma reconstruction in
their Fig. 1, instead of their own, seeing that the purpose of their
comment appears to be advancing the issue of assessing alternative
reconstructions.
We strongly disagree that our computed ocean floor area aged 1–65
m.y. at 100 Ma is grossly overestimated. We compare three alternative
reconstructions for 100 Ma since 2008 (Figs. 1B–1D).
We are curious to know how Norton and Lawver derived their
substantially lower estimate for the 100 Ma ridge flank area, and
note that the sole evidence for both their 200 and 100 Ma “alternative reconstructions” provided to the reader is a blue and a green
dot, respectively, in the inset diagram in their Fig. 1 (reproduced
from our paper). If they had provided alternative age grids for these
times, then we would be in a position to assess the differences between their reconstructions and ours.
Irrespective of whether we adopt our revised 200 Ma ridge-flank
area estimate (Fig. 1A) or that by Norton and Lawver, our 200 Ma
to present fluctuations in the area of young mid-ocean ridge flanks
do not change dramatically. If we accepted, for argument’s sake,
Norton and Lawver’s mysterious 100 Ma ridge flank area estimate,
then the present-day ocean floor area aged 1–65 Ma is still only
about 70% of their 100 Ma value (their figure 1 inset), far from
being “almost constant through time.” Therefore, our inference of a
dramatic decrease of the area of young mid-ocean ridge flanks since
the mid-Cretaceous still stands even in the scenario promoted by
Norton and Lawver. That it should not be constant has a firm underpinning provided by fully dynamic mantle convection models
(Coltice et al., 2012; 2013). They illustrate that over a Wilson cycle
there are variations by a factor of 2 in the rate of production of new
seafloor, with concomitant major changes in the area-age distribution of the seafloor. Their models support that a stable supercontinent is accompanied by a rectangular age-area distribution (Fig.
1A), with breakup and dispersal leading to a skewed distribution
(Figs. 1B–1D), reflecting the progressive creation of new crust at
the expense of older crust being subducted, while the triangular
distribution we observe today reflects a near constant production of
oceanic lithosphere compared to what is destroyed (Coltice et al.,
2013). Therefore, the main conclusions of our paper are robust,
supported by independent geodynamic models, and not dependent
on geological uncertainties.
ACKNOWLEDGMENTS
Figure 1. A: Revised paleo-age grid for 200 Ma. B–D: Alternative paleo-age
grids for 100 Ma, representing the “geological uncertainty” involved in
alternative mid-ocean ridge geometries and back-arc basins. B: Müller et al.
(2008). C: Müller et al . (2013). D: This paper (Robinson projection). Grayshaded transparent regions in the revised 100 Ma reconstruction (D)
illustrate the conjugate ridge flank areas constrained by magnetic anomaly
identifications. Inset age-area histograms in 20 m.y. bins (x-axis: 0–200 m.y.
old crust, y-axis: 0–35%) illustrate the rectangular age-area distribution
during supercontinent stability (A) and skewed seafloor distributions during
supercontinent dispersal (B–D).
The oldest reconstruction (Müller et al., 2008) results in an ocean floor
area aged 1–65 m.y. of 269 × 106 km2 (Fig. 1B), as compared to 251 ×
106 km2 of that by Müller et al. (2013) (Fig. 1C) and 253 × 106 km2 of
our most recent reconstruction (Fig. 1D). The reconstructions reflect the
stepwise inclusion of more complex mid-ocean ridge systems, modelling
the breakup of the Ontong Java, Manihiki and Hikurangi plateaus (Fig.
1C), the subsequent inclusion of back-arc basins in the eastern Tethys
and Southeast Asia, following Zahirovic et al. (2013), and some
additional changes to spreading ridges in the Indian and Pacific oceans
(Fig. 1D). Even though these reconstructions have undergone substantial
changes through time, the seafloor age histograms, mean ages, and ridge
flank areas less than 65 m.y. old have only undergone relatively minor
changes (Figs. 1B–1D).
This research was funded by Australian Research Council grants
FL0992245, DP0987713, and DP0987604: and by the Research Council of
Norway through its Centres of excellence funding scheme, Project 223272.
We are grateful for Nicky Wright’s help with preparing Figure 1.
REFERENCES CITED
Coltice, N., Rolf, T., Tackley, P., and Labrosse, S., 2012, Dynamic causes of
the relation between area and age of the ocean floor: Science, v. 336, p.
335-338.
Coltice, N., Seton, M., Rolf, T., Müller, R., and Tackley, P.J., 2013, Convergence of tectonic reconstructions and mantle convection models for significant fluctuations in seafloor spreading: Earth and Planetary Science
Letters, v. 383, p. 92-100.
Müller, R., Dutkiewicz, A., Seton, M., and Gaina, C., 2013, Seawater chemistry driven by supercontinent assembly, breakup, and dispersal: Geology,
41, p. 907-910.
v. R.D., Sdrolias, M., Gaina, C., Steinberger, B., and Heine, C., 2008,
Müller,
Long-term sea level fluctuations driven by ocean basin dynamics: Science, v. 319, p. 1357-1362.
Norton, I.O., and Lawver, L.A., 2014, Comment: Seawater chemistry driven
by supercontinent assembly, breakup, and dispersal: Geology, vol. 42, p.
e334.
Shephard, G.E., Müller, R.D., and Seton, M., 2013, The tectonic evolution
of the Arctic since Pangea breakup: Integrating constraints from surface
geology and geophysics with mantle structure: Earth-Science Reviews, v.
124, p. 148-183.
Zahirovic, S., Seton, M., and Müller, R., 2013, The Cretaceous and Cenozoic tectonic evolution of Southeast Asia: Solid Earth Discussions, v. 5,
doi:10.5194/sed-5-1335-2013.
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