Organics found on mars!!!

[KerikBalm]

Chalk is made of calcium carbonate, and often contains such, but there's nothing special about calcium carbonate, and without looking at the microscopic details, you can't tell if its abiotic.

If you are seeing accumulations of exoskeletons, you're basically asking to find fossils, which are much clearer signs of life than even DNA (unless the DNA molecule is really long - short nucleic acids can also form abiotically).

"Chalk is composed mostly of calcium carbonate with minor amounts of silt and clay. It is normally formed underwater, commonly on the sea bed, then consolidated and compressed during diagenesis into the form commonly seen today. During diagenesis silica accumulates to form chert or flint nodules within the carbonate rock."