Trace metal and isotopic ratios, including some rare earth elements, Mg/Ca, manganese and strontium concentrations, δ¹⁸O, δ¹³C, and ⁸⁷Sr/⁸⁶Sr, were analyzed in the carbonate cements from 17 Phanerozoic carbonate hardgrounds. The sensitivity of the geochemical signal to alteration depends on the geochemical analysis in question and the environmental water-rock ratio. Of these samples, only our modern sample has measurements consistent with primary precipitation from seawater; all other samples precipitated from chemically evolved seawater or were influenced by meteoric water, even if only minimally changed. The more recent samples from the Cenozoic had seawater ⁸⁷Sr/⁸⁶Sr. The Mesozoic samples, in contrast, did not preserve seawater ⁸⁷Sr/⁸⁶Sr, even though the Mg/Ca, δ¹⁸O, and δ¹³C values were consistent with precipitation from seawater. Finally, the Paleozoic samples preserved expected seawater ⁸⁷Sr/⁸⁶Sr, though REE and δ¹⁸O suggest primary precipitation was from evolved seawater. Additionally, we place our results in the context of open vs. closed system precipitation using transects of the Mg/Ca ratios across individual cements. Overall, we stress that one geochemical measurement provides only a partial record of fluid composition, but multiple measurements allow a potential understanding of the seawater geochemical signal.