▁bro ader ▁r idge ▁with ▁decre ased ▁average ▁depth , ▁taking ▁up ▁more ▁space ▁in ▁the ▁ocean ▁bas in . ▁This ▁disp laces ▁the ▁over lying ▁ocean ▁and ▁causes ▁sea ▁levels ▁to ▁rise . ▁ ▁Se ale vel ▁change ▁can ▁be ▁attributed ▁to ▁other ▁factors ▁( ther mal ▁expansion , ▁ice ▁mel ting , ▁and ▁mant le ▁con ve ction ▁creating ▁dynamic ▁top ography ). ▁Over ▁very ▁long ▁times cal es , ▁however , ▁it ▁is ▁the ▁result ▁of ▁changes ▁in ▁the ▁volume ▁of ▁the ▁ocean ▁bas ins ▁which ▁are , ▁in ▁turn , ▁affected ▁by ▁rates ▁of ▁se af loor ▁spread ing ▁along ▁the ▁mid - o cean ▁rid ges . ▁ ▁The ▁high ▁se ale vel ▁that ▁occurred ▁during ▁the ▁C ret ace ous ▁Period ▁( 1 4 4 – 6 5 ▁Ma ) ▁can ▁only ▁be ▁attributed ▁to ▁plate ▁t ect on ics ▁since ▁thermal ▁expansion ▁and ▁the ▁absence ▁of ▁ice ▁sheets ▁by ▁themselves ▁cannot ▁account ▁for ▁the ▁fact ▁that ▁sea ▁levels ▁were ▁ 1 0 0 – 1 7 0 ▁meters ▁higher ▁than ▁today . ▁ ▁Imp act ▁on ▁se aw ater ▁chem istry ▁and ▁carbon ate ▁de position ▁▁ ▁Se af loor ▁spread ing ▁on ▁mid - o cean ▁rid ges ▁is ▁a ▁global ▁scale ▁ion - exchange ▁system . ▁H ydro ther mal ▁v ents ▁at ▁spread ing ▁cent ers ▁introduce ▁various ▁amounts ▁of ▁iron , ▁sul fur , ▁mang an ese , ▁sil icon ▁and ▁other ▁elements ▁into ▁the ▁ocean , ▁some ▁of ▁which ▁are ▁rec