User:Kökkerö/sandbox

= Carbonate platform =

Drowning
Drowning of carbonate platform is an event where the relative sea level rise is faster than the accumulation rate on a carbonate platform, which eventually leads to the platform to submerge below the euphotic zone. In the geologic record of a drowned carbonate platform, neritic deposits change rapidly into deep-marine sediments. Typically hardgrounds with ferromanganese oxides, phosphate or glauconite crusts lie in between of neritic and deep-marine sediments.

Several drowned carbonate platforms have been found in the geologic record. However, it has not been very clear how the drowning of carbonate platforms exactly happen. Modern carbonate platforms and reefs are estimated to grow approximately 1 000 μm/yr, possibly several times faster in the past based on geological record. 1 000 μm/yr growth rate of carbonates exceeds by orders of magnitude any relative sea level rise that is caused by long-term subsidence, or changes in eustatic sea level. Based on the rates of these processes, drowning of the carbonate platforms should not be possible, which causes "the paradox of drowned carbonate platforms and reefs".

Since drowning of carbonate platforms requires exceptional rise in the relative sea level, only limited number of processes can cause it. According to Schlager only anomalously quick rise of relative sea level or benthic growth reduction caused by deteriorating changes in the environment could explain the drowning of the platforms. For instance regional downfaulting, submarine volcanism or glacioeustacy could be the reason for rapid rise in relative sea level, whereas for example changes in oceanic salinity might cause the environment to become deteriorative for the carbonate producers.

One example of a drowned carbonate platform is located in Huon Gulf, Papua New Guinea. It is believed to be drowned by rapid sea level rise caused by deglaciation and subsidence of the platform, which enabled coralline algal-foraminiferal nodule and halimeda limestones to cover the coral reefs.

Plate movements carrying carbonate platforms to latitudes unfavorable for carbonate production are also suggested to be one of the possible reasons for drowning. For example, guyots located in the Pacific Basin between Hawaiian and Mariana Islands are believed to be transported to low sourthern latitudes (0-10°S) where equatorial upwelling occurred. High amounts of nutrients and higher productivity caused decrease in water transparency and increase in bio-eroders populations, which reduced carbonate accumulation and eventually led to drowning.