▁would ▁have ▁been ▁low - M g ▁cal cite ▁in ▁past ▁cal cite ▁se as , ▁meaning ▁that ▁the ▁M g / Ca ▁ratio ▁in ▁an ▁organ ism ' s ▁ske leton ▁var ies ▁with ▁the ▁M g / Ca ▁ratio ▁of ▁the ▁se aw ater ▁in ▁which ▁it ▁was ▁grown . ▁ ▁The ▁min eral ogy ▁of ▁re ef - building ▁and ▁sed iment - produ cing ▁organ isms ▁is ▁thus ▁reg ulated ▁by ▁chemical ▁re actions ▁occurr ing ▁along ▁the ▁mid - o cean ▁r idge , ▁the ▁rate ▁of ▁which ▁is ▁controlled ▁by ▁the ▁rate ▁of ▁sea - floor ▁spread ing . ▁ ▁History ▁ ▁Disc overy ▁ ▁The ▁first ▁indic ations ▁that ▁a ▁r idge ▁bis ect s ▁the ▁Atlantic ▁Ocean ▁bas in ▁came ▁from ▁the ▁results ▁of ▁the ▁British ▁Ch allen ger ▁expedition ▁in ▁the ▁nin ete enth ▁century . ▁Sound ings ▁from ▁lines ▁dropped ▁to ▁the ▁se af loor ▁were ▁analyz ed ▁by ▁ocean ograph ers ▁Matthew ▁Font aine ▁Ma ury ▁and ▁Charles ▁Wy ville ▁Thom son ▁and ▁revealed ▁a ▁prominent ▁rise ▁in ▁the ▁se af loor ▁that ▁ran ▁down ▁the ▁Atlantic ▁bas in ▁from ▁north ▁to ▁south . ▁Son ar ▁echo ▁sound ers ▁confirmed ▁this ▁in ▁the ▁early ▁tw enti eth ▁century . ▁ ▁It ▁was ▁not ▁until ▁after ▁World ▁War ▁II , ▁when ▁the ▁ocean ▁floor ▁was ▁survey ed ▁in ▁more ▁detail , ▁that ▁the ▁full ▁extent ▁of ▁mid - o cean ▁rid ges ▁became ▁known . ▁The ▁V ema , ▁a ▁ship ▁of ▁the