For one, the killing of the preserved mammoths by dust storms. Those that were found largely intact showed no signs of distress at their deaths. It appears something almost-instantly killed them, without warning. I'm not ruling out some electrical event, but a dust storm wouldn't've killed them so quickly, & they would have shown some signs of distress, or some escape attempt, and their mouths & lungs woulda been fulla dust.
As for ice ages, there's evidence that four of them have occurred, with some mini-ages in between. There may well have been such an age after the flood, but one thing against this is that God told Noah & crew to be fruitful & multiply, and animals quickly spread all over earth.
While Prof. Oard has done some commendable research, he's so-focused on trying to establish a "young earth" that he's overlooked some equally-painstaking research by others, & has added his imagination to the FACTS.
I'm not at all saying there weren't meny species of animals that didn't become extinct after the flood. behemoth may well have been one. There's no known critter now fitting its description, especially an herbivore with a large, powerful tail. It coulda been a megatherium, the largest of the ground sloths, which could reach 20 feet in length, able to stand 15 ft. high, was a herbivore, & had a fairly-short, but powerful tail. There's no explanation as to why this well-adapted creature became extinct.
As for leviathan, I believe it was either a salt-water croc, which can grow to well over 20 ft. long, or some aquatic sauropod such as tylosaurus that survived the flood. (It's unknown if tylo had scales.)
(BTW, did you know ancient alligators could reach a length of 50 feet, & the first horses were no bigger than collies? No specimans of either of those animals of those sizes have been seen by man at any time. Howdya explain those positively-verified FACTS?)
Old earthers like you have about sixty theories of the Ice Age. Oard is an expert on the subject, but you have put forth no old earther expert. Oard has silenced a lot of them after forty years of research. Darwin and deep time have massive scientific problems.