I think its a downtuned E to a D, but I'm not sure. If it sounds better to you,
play a 6-8 instead of 8-10 in the second part and then open E.
G|----7-7p5-------------------------------|
D|----------7-----------------------------|X4
A|-5----------8-10-8-10-8-5---------------|
D|----------------------------------------|
[ Tab from: https://www.guitartabs.cc/tabs/l/led_zeppelin/moby_dick_btab_ver_3.html ]
G|----------------------------------------|
D|--5-5p3-0-------------------------------|X2
A|----------------------------------------|
D|----------8-10-8-10-8-0-----------------|
G|----7-7p5-------------------------------|
D|----------7-----------------------------|X2
A|-5----------8-10-8-10-8-5---------------|
D|----------------------------------------|
G|-----------------------------------7-5-3-------|
D|-7-7-5-0-7-7-0-5-7-5-5-5-5-5-3-0---------5-3-0-|
A|-----------------------------------------------|
D|-----------------------------------------------|
