Hello,
If you are really keen in pursuing phd in earth and ocean science then I would suggest you to pursue master of technology in sath and environmental science . It is a post graduate environmental science course . It is an inter disciplinary field of studies ,darling with environmental problems geological processes as well as human interactions.
The earth and environment science are geology, ocean studies, earth studies,etc. The program includes studying about earths component material ,development of its structure,the process by which these change with time and many more .
The eligibility criteria for the same is be or btech or any equivalent degree with minimum 50 % marks or equivakent cgpa .
Some of the reputed colleges and universities conduct entrance exam for the same.
One of the most reputed colleges which offers this course is -
Indian Institute of Technology - IIT Kharagpur
Hope this helps.
Good luck