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"Yesterday at 5:52pm my beautiful Grandma Carol Baldwin, the matriarch of the Baldwin family completed her journey here on earth," Bieber captioned a vintage photograph of Carol, who was 92 at the time of her death.
"Today I celebrate her, the life she lived, and the legacy she leaves behind," the star added. "We love you. 🤍🕊."
Alec, 64, announced his mother's death on Thursday by posting a throwback photo of her positioned beside a more recent image on Instagram.
In the caption, he wrote, "Carol M. Baldwin, mother of actors Alec, Daniel, William and Stephen Baldwin and two daughters, Elizabeth and Jane, died today in Syracuse, New York."
"Born Carol Newcomb Martineau, in Syracuse on December 15, 1929, she was the child of Roy and Marion Martineau and had one brother, Daniel, and five sisters, Patricia, Joan, Diane, Louise and Rebecca," Alec added.
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The actor said she met her husband Alexander R. Baldwin Jr., who was nicknamed Alec, when they were students at Syracuse University. The family moved to Long Island, and while there, he worked as a history teacher and football and riflery coach at Massapequa High School. He later died in 1983 at age 55, his son noted in the post.
"Carol Baldwin raised her children and, when her last was a teenager, went to work in marketing research for a firm called Quick Test, operating out of a local shopping mall," Alec continued. "In 1991, she was diagnosed with breast cancer. After she survived, she joined with a group of Long Island-wide breast cancer support groups to launch an effort with SUNY Stony Brook."
The Baldwin matriarch later partnered with the school's president at the time, Shirley Strum Kenny, to launch the Carol M. Baldwin Breast Cancer Research Fund at the Stony Brook location, which was followed by a branch at SUNY Upstate University. "The combined efforts have raised millions of dollars for the cause," Alec wrote.
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In a statement included in the touching Instagram post, Alec shared, "My mother taught me about second acts. And third ones, too."
He added: "She spent the last 25 years of her life as a fighter and a champion for the cause to which she devoted her life. We are all enormously proud of her accomplishments."
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Bieber, who is the daughter of Stephen, 56, previously shared Alec's post on her Instagram Story and wrote, "We love you Grandma." She added emojis of a white heart and a dove.
Carol is survived by her six children, 25 grandchildren, and 14 great-grandchildren. Details of a memorial will be shared at a later time.