MODERN NON-TROPICAL CARBONATES CALIBRATED AGAINST NUTRIENTS AND TEMPERATURE

In order to calibrate modern carbonate formation against nutrients and temperature in non-tropical settings we selected the marginal ocean basin of the Gulf of California (GOC), Mexico, as a unique modern example encompassing a complete suite of temperate to subtropical carbonate environments along a latitudinal transect spanning the temperate to subtropical realm (30ºN to 23ºN). We sampled carbonates to 120m water depth and conducted high-resolution (hourly) in situ time series monitoring of the above mentioned oceanographic parameters at several stations along the entire western GOC margin. From south to north carbonate production is characterized by:

(1) subtropical coral dominated shallow carbonate factories (0-20 m water depth) with average SST’s of 25ºC (min. 18ºC, max. 31ºC) and chlorophyll a levels, which are a proxy for nutrients, of 0.3 mg Cla/m^-3 (max. 0.48, min. 0.1).

(2) warm-temperate red algal dominated subtidal to inner shelf carbonate production (5-40m) in the central GOC exhibiting average temperatures of 23ºC (min. 18ºC, max. 30ºC) and chlorophyll a levels of 0.6 Cla/m^-3 (max. 5.62, min. 0).

(3) cool-temperate type molluskan-bryozoan rich outer shelf factories in the northern GOC (20- >120m) with SST’s of only 20ºC (min. 13ºC, max 29ºC) and average contents of 2.2 mg Cla/m^-3 (max. 8.38, min. 0).

With an average of 35‰, salinity fluctuates little throughout the year and shows a small gradient between southern and northern sites, and therefore does not excert a large controlling factor on carbonate producers in the GOC. By calibrating sedimentological with in situ measured oceanographic information in different environments within the same oceanic setting we provide a modern example for interpretation of similar non-tropical fossil carbonate settings, and we demonstrate the importance of recognizing not only the average, but also short term extreme values of temperature and often neglected trophic resources as an important factor determining the composition of carbonates.