这句英语的一个that从句难倒我了,

这句英语的一个that从句难倒我了,
This indicates that the source of the sediments and predominant post-depositional
processes have remained rela-tively unchanged during the approximately 700
years that the upper 2 m have been accumulating.
这句话的大部分我能理解,我不是要求你们全文翻译,这句话关键是最下面那行的year后面的that,它不像是修饰year这个单词的定语从句的开头,那个during也不能搅到后面的从句里面来,我把我理解的意思先写出来.
This indicates that the source of the sediments and predominant post-depositional
这表明了沉积物（sediments）的来源和主要的沉积后（post-depositional
processes have remained relatively unchanged during the approximately 700 years
在大约700年的时间里,保持相对的不变
that the upper 2 m have been accumulating.
在这700年里,最上面的2m一直在累积.
这段翻译就是最后那个在这700年里那句我觉得不妥,that不像是year的定语,作定语的话少了个介词,那么它到底是什么呢?