Aragonite Seas Research Articles

Secular variations in Precambrian seawater chemistry and the timing of Precambrian aragonite seas and calcite seas: Comment and Reply

In her comment on my paper on secular variations in Precambrian seawater chemistry, Dr. Sumner has argued that aragonite pseudomorphs are abundant in the 2.5–2.6 Ga Campbellrand-Malmani carbonates of the Transvaal Basin, South Africa, occurring mainly as fibrous cements replaced by calcite

Secular variations in Precambrian seawater chemistry and the timing of Precambrian aragonite seas and calcite seas: Comment and Reply

[Hardie (2003)][1] identified intervals of “aragonite” and “calcite seas” by modeling Mg/Ca ratios in Neoarchean to Neoproterozoic seawater using estimates of hydrothermal and weathering fluxes of key ions. His results agree with most published reports of aragonite pseudomorphs with the

Secular variations in Precambrian seawater chemistry and the timing of Precambrian aragonite seas and calcite seas

An extension of the seawater secular-variation model that successfully predicted the observed timing of Phanerozoic MgSO 4 vs. KCl marine evaporites and aragonite seas vs. calcite seas has been applied to the prediction of the secular variations in the major ion chemistry of seawater and aragonite seas vs. calcite seas during the Precambrian. Testing of the predictions was based on those Precambrian seafloor carbonate precipitates that have been interpreted by others to have formed originally as aragonite. Of 16 examples of Precambrian seafloor aragonite, 14 fall within the 6 periods of aragonite seas predicted for the Late Archean–Proterozoic by the model, 1 falls on the transition between an aragonite sea and calcite sea, and 1 falls in a period of calcite seas. This strong correlation supports the following predictions of the model: (1) Precambrian seawater was a saline NaCl water with Ca > HCO 3 since at least the Late Archean. (2) The major ion compositions of Precambrian seawater chemistry and their secular variations are in the same ranges as those of Phanerozoic seawater. (3) The Mg/Ca mole ratio in seawater has controlled the types of CaCO 3 polymorphs that have precipitated from Earth9s oceans throughout the Phanerozoic and most, if not all, of the Precambrian.

The origin and significance of zigzag microstructure in late Paleozoic Lophophyllidium (Anthozoa, Rugosa)

In late Paleozoic solitary Rugosa, the zigzag microstructure as defined by Schindewolf (1942) is related to presence of an elevated magnesium content within biogenic calcite (intermediate magnesian calcite, IMC) and its subsequent loss during diagenesis by microdissolution and neomorphism. This particular microstructure has been recognized with certainty only in some Carboniferous and Permian rugose corals (e.g., Lophophyllidium spp.). Septal and other skeletal microstructures in those corals are dominantly (oblique) sloping-lamellar, which is also interpreted as diagenetic in origin. Two directions of oblique lamellae commonly occur in thickened skeletal elements, forming chevrons that make up zigzag microstructure with its orientation determined by presence of microdolomite blebs within skeletal calcite. Geochemical studies of corals from the Mississippian Imo Formation of Arkansas, the Pennsylvanian Buckhorn asphalt of Oklahoma and Pennsylvanian Kendrick Shale of Kentucky all indicate that magnesium content in skeletal calcite of the corals was elevated, with a maximum in the neighborhood of six to eight mole percent CaCO3, thereby forming intermediate magnesium calcite. Corals with this zigzag microstructure apparently only occurred during the late Paleozoic interval of “aragonite seas”; as a result, this diagenetic behavior of rugose corals can serve as a proxy for secular change in marine chemistry and/or climate.

Zigzag microstructure in rugose corals: A possible indicator of relative seawater Mg/Ca ratios

Low-Mg calcite (LMC) and high-Mg calcite (HMC) have been identified confidently as original biominerals in different rugose corals. Devonian Tabulophyllum had a LMC skeleton, whereas Pennsylvanian Lophophyllidium had a HMC skeleton. Zigzag microstructure, a diagenetic microstructure in rugose corals, may be related to early migration of Mg2+ ions during transformation of HMC into LMC. Therefore, zigzag microstructure may indicate that the original mineral was HMC even in specimens that subsequently converted to LMC. Stratigraphically, corals with zigzag microstructure are confined to the late Paleozoic, which is coincident with the “aragonite sea” of P.A. Sandberg. Hence, fluctuating Mg/Ca ratios may have directly controlled oscillations in marine carbonate chemistry and the mineral composition of rugose corals so that HMC skeletons (as indicated by zigzag microstructure) formed during intervals with “aragonite seas” and LMC skeletons (e.g., Tabulophyllum ) formed during intervals with “calcite seas.”

Secular oscillations in the carbonate mineralogy of reef-building and sediment-producing organisms driven by tectonically forced shifts in seawater chemistry

The primary mineralogy of oolites and early marine carbonate cements led Sandberg [Nature 305 (1983), 19–22] to divide the Phanerozoic Eon into three intervals of `aragonite seas' and two intervals of `calcite seas'. Hardie [Geology 24 (1996), 279–283] has shown that these oscillations, together with synchronous oscillations in the mineralogy of marine potash evaporites, can be explained by secular shifts in the Mg/Ca ratio of seawater driven by changes in spreading rates along mid-ocean ridges. The Hardie model also predicts that high-Mg calcite should precipitate along with aragonite, as it does in today's aragonite sea. We have uncovered oscillations in the carbonate mineralogy of hypercalcifying organisms (ones that have produced massive skeletons, large reefs, or voluminous bodies of sediment) that correspond to Sandberg's aragonite seas and calcite seas and that are predicted by the Hardie model. Particular groups of corals, sponges, and algae appear to have been dominant reef builders only when favored by an appropriate Mg/Ca ratio in seawater. In early and middle Paleozoic calcite seas (Calcite I), reefs were dominated by calcitic tabulate, heliolitid, and rugose corals and calcitic stromatoporoids. In contrast, during the period of late Paleozoic–early Mesozoic aragonite seas (Aragonite II), aragonitic groups of sponges, scleractinian corals, and phylloid algae, as well as high-Mg calcitic red algae, were principal reef builders. During Late Cretaceous time, at the acme of Calcite II, massive rudists displaced aragonitic hermatypic corals. In today's aragonite sea (Aragonite III) scleractinian corals are again dominant reef builders, along with high-Mg calcitic coralline algae. Major sediment-producing algae exhibit temporal patterns similar to those of reef builders. Calcitic receptaculitids flourished during Calcite I, whereas aragonitic dasycladaceans did not become dominant rock formers until Aragonite II. During Calcite II, calcitic nannoplankton formed massive coccolith chalks in warm shallow seas of the Late Cretaceous, after the Mg/Ca ratio of seawater had reached a very low value and calcium concentration, a very high value. As the Mg/Ca ratio of seawater rose and calcium concentration fell during the Cenozoic Era, individual coccoliths, on average, became less massive and encrusted cells less thickly. By Pliocene time, during Aragonite III, the prominent genus Discoaster secreted only narrow-rayed coccoliths that covered less than 25% of the cell surface. Also during Aragonite III, the aragonitic green alga Halimeda emerged as the dominant skeletal sediment producer in reef tracts. The influence of seawater chemistry on skeletal secretion appears to have been especially strong for morphologically simple taxa that exert relatively weak control over their own calcification. Such groups include algae, sponges, corals, and bryozoans. Morphological simplicity also permits these groups to adopt vegetative or colonial modes of growth that confer success in competition for space on reefs. This linkage, in addition to the basic chemical demands of hypercalcification, has given the Mg/Ca ratio of seawater strong control over the success of individual reef-building taxa. More generally, this ratio appears to have strongly influenced evolutionary changes in the skeletal mineralogy of sponges and cheilostome bryozoans throughout their history. We conclude that throughout Phanerozoic time a chain of causation has extended from mid-ocean ridge processes, via seawater chemistry, to the mineralogical and biological composition of reef communities and bioclastic carbonate deposits.

Secular variation in seawater chemistry: An explanation for the coupled secular variation in the mineralogies of marine limestones and potash evaporites over the past 600 m.y.

Secular changes in the mineralogies of marine nonskeletal limestones and potash evaporites occur in phase on a 100–200 m.y. time scale such that periods of “aragonite seas” are synchronized with MgSO4 evaporites and periods of “calcite seas” with KCl evaporites. It is proposed that these coupled changes are the result of secular variation in seawater chemistry controlled primarily by fluctuations in the mid-ocean ridge hydrothermal brine flux, which in turn have been driven by fluctuations in the rate of ocean crust production. Quantitative predictions based on this hypothesis yield secular variation in limestone and potash evaporite mineralogies that closely match the observed variation over the past 600 m.y., providing strong support for the thesis that seawater chemistry, rather than remaining constant, has oscillated significantly over geologic time.

Some aspects of the oceanography of Little Port Walter Estuary, Baranof Island, Alaska : Powers C.F., 1963. U.S. Fish. Wild. Serv., Fish. Bull., 63 (1): 143–164

The primary mineralogy of oolites and early marine carbonate cements led Sandberg [Nature 305 (1983), 19–22] to divide the Phanerozoic Eon into three intervals of `aragonite seas' and two intervals of `calcite seas'. Hardie [Geology 24 (1996), 279–283] has shown that these oscillations, together with synchronous oscillations in the mineralogy of marine potash evaporites, can be explained by secular shifts in the Mg/Ca ratio of seawater driven by changes in spreading rates along mid-ocean ridges. The Hardie model also predicts that high-Mg calcite should precipitate along with aragonite, as it does in today's aragonite sea. We have uncovered oscillations in the carbonate mineralogy of hypercalcifying organisms (ones that have produced massive skeletons, large reefs, or voluminous bodies of sediment) that correspond to Sandberg's aragonite seas and calcite seas and that are predicted by the Hardie model. Particular groups of corals, sponges, and algae appear to have been dominant reef builders only when favored by an appropriate Mg/Ca ratio in seawater. In early and middle Paleozoic calcite seas (Calcite I), reefs were dominated by calcitic tabulate, heliolitid, and rugose corals and calcitic stromatoporoids. In contrast, during the period of late Paleozoic–early Mesozoic aragonite seas (Aragonite II), aragonitic groups of sponges, scleractinian corals, and phylloid algae, as well as high-Mg calcitic red algae, were principal reef builders. During Late Cretaceous time, at the acme of Calcite II, massive rudists displaced aragonitic hermatypic corals. In today's aragonite sea (Aragonite III) scleractinian corals are again dominant reef builders, along with high-Mg calcitic coralline algae. Major sediment-producing algae exhibit temporal patterns similar to those of reef builders. Calcitic receptaculitids flourished during Calcite I, whereas aragonitic dasycladaceans did not become dominant rock formers until Aragonite II. During Calcite II, calcitic nannoplankton formed massive coccolith chalks in warm shallow seas of the Late Cretaceous, after the Mg/Ca ratio of seawater had reached a very low value and calcium concentration, a very high value. As the Mg/Ca ratio of seawater rose and calcium concentration fell during the Cenozoic Era, individual coccoliths, on average, became less massive and encrusted cells less thickly. By Pliocene time, during Aragonite III, the prominent genus Discoaster secreted only narrow-rayed coccoliths that covered less than 25% of the cell surface. Also during Aragonite III, the aragonitic green alga Halimeda emerged as the dominant skeletal sediment producer in reef tracts. The influence of seawater chemistry on skeletal secretion appears to have been especially strong for morphologically simple taxa that exert relatively weak control over their own calcification. Such groups include algae, sponges, corals, and bryozoans. Morphological simplicity also permits these groups to adopt vegetative or colonial modes of growth that confer success in competition for space on reefs. This linkage, in addition to the basic chemical demands of hypercalcification, has given the Mg/Ca ratio of seawater strong control over the success of individual reef-building taxa. More generally, this ratio appears to have strongly influenced evolutionary changes in the skeletal mineralogy of sponges and cheilostome bryozoans throughout their history. We conclude that throughout Phanerozoic time a chain of causation has extended from mid-ocean ridge processes, via seawater chemistry, to the mineralogical and biological composition of reef communities and bioclastic carbonate deposits.