Growth and high-resolution paleoenvironmental signals rhodoliths (coralline red algae): A new biogenic archive

We investigated rhodoliths (coralline red algae) from a subtropical ocality in the Gulf of California (Lithotharnniurn crassiusculurn) and a subarctic locality in Newfoundland (Lithotharnniurn glaciale) for their potential as paleoenvironmental rchives using microanalytical geochemical techniques tomeasure variations in ;5•80, Mg, and Ca. Rhodoliths are potentially well suited as recorders of shallow water paleoenvironmental signals because they (1) have worldwide distribution from the tropics to polar regions, (2) are long lived from decades to centuries, and (3) display well-developed growth bands. Our results indicate that rhodolith growth bands preserve ultrahigh-resolution records of paleoceanographicpaleoclimatic hange and likely constitute an important new archive for reconstructing the paleoenvironmental history of littoral-neritic areas in which these algae are found. The ;5180 content of individually sampled rhodolith growth bands ranges from -2.4 to -4.6 %o in L. crassiusculurn a d from-3.2 to -0.3 %o in L. glaciale. In both cases, the range of ;5•80 values suggests a slightly lower amplitude of variation in sea surface temperature than that actually measured in the ocean at the two study sites. Both L. crassiusculurn and L. glaciale show a negative offset from isotopic equilibrium. Electron microprobe analysis of magnesium and calcium in growth bands reveals cyclic variations with values ranging between 7.7-18.5 mol % MgCO3 in L. glaciale and 13.2-22.5 mol % MgCO3 in L. crassiusculurn. In addition, electron microprobe lement maps highlight individual growth bands, provide a powerful approach to study rhodolith formation, and indicate that the specimens we analyzed have vertical growth rates of 250-450 •tm/yr.