Gravitational Potential Energy per Unit Area as a Constraint on Archean Sea Level. (30th October 2018)

• Record Type: Journal Article
• Authors: Molnar, Peter
• Is Part Of: Geochemistry, geophysics, geosystems. Volume 19:Number 10(2018)
• Page Start: 4063
• Page End: 4095
• Publication Date: 2018-10-30
• Subjects: gravitational potential energy -- Archean lithosphere -- Archean sea level; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 550.5
• DOI: 10.1029/2018GC007766
• Languages: English
• ISSNs: 1525-2027

Abstract

Abstract: To constrain the density structure of Archean lithosphere, I assume that gravitational potential energy (GPE) per unit area at Archean spreading centers equals that for continents whose surfaces lay near sea level. The present‐day balance of GPE limits average density deficits in Archean mantle lithosphere due to depletion of iron during its formation to ∼30–45 kg/m 3 . For an Archean volume of seawater equal to or greater than that today and heat loss approximately three times that today (e.g., at ∼3.5 Ga), plausible Archean structures, constrained by GPE balance, favor submerged spreading centers and continents. Emergent Archean continents would be allowed by a combination of the following (a) heat loss only twice that today, (b) less than approximately half of the present‐day volume of thick crust, either continental or oceanic plateaus, and (c) deep Archean spreading centers ( ≳ 2 km below continents), which would require a thickness of Archean oceanic crust ≲ 25 km. If the volume of Archean seawater were 50% greater than that today, emergent spreading centers could have existed only as isolated unusual features, and emergent continents would require both limited extent of thick crust and spreading centers deeper that ∼2 km. The observed widespread development of continental margins and carbonate platforms at ∼2.7–2.5 Ga is consistent with heat loss roughly twice that today, a volume of seawater comparable with that today, and either (a) a volume of continental crust comparable to that at present and oceanic crust ≲ 30 km thick or (b) thicker oceanic crust, possibly 40–50 km, but with a volume of continental crust less than approximately half that today. Calculated temperatures at the Archean Moho are ∼700–900 °C. These calculations do not rule out a hot early Archean ocean (∼50–80 °C) and do not favor an early Archean emergence of life on land above sea level. Key Points: Equating gravitational potential energy beneath spreading centers and continents constrains chemical density anomalies Rare emergent early Archean continents constrain heat loss to approximately three times that today Emergence of continents at ∼2.5 Ga is consistent with heat loss of twice that today