Snowball Earth - Snowball Earth - Wikipedia

Proterozoyning qor to'pi davrlari
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 Neoproterozoy davr
 Snowball Earth
Taxminiy Proterozoy muzlik davrlari.[2][1] Oldingi uchrashuvGaskiers muzliklar noaniq. Kaygalarga kelsak, uning mavjudligiga ba'zilar shubha qilmoqda. Oldindan va uzoqroq davom etishi mumkin bo'lgan qor to'pi bosqichi Huron muzligi, ko'rsatilmagan.

The Snowball Earth gipoteza bir yoki bir nechta Yerning paytida buni taklif qiladi muzxona iqlim, Yer yuzasi butunlay yoki deyarli muzlatilgan bo'lib, 650 yildan biroz oldinroq Mya (million yil oldin) davrida Kriyogen davri. Gipoteza tarafdorlari buni eng yaxshi tushuntiradi, deb ta'kidlaydilar cho'kindi odatda depozitlar muzlik kelib chiqishi at tropik paleoolatlilik va boshqa sirli xususiyatlar geologik yozuv. Gipotezaning muxoliflari geologik dalillarning global muzlik uchun ta'sirini va geofizik maqsadga muvofiqligi muz - yoki shilimshiq - yopiq okean[3][4] va butunlay muzlatilgan holatdan qochish qiyinligini ta'kidlang. Bir qator javobsiz savollar, shu jumladan Yer to'la qor to'pi bo'lganmi yoki ochiq (yoki mavsumiy ochiq) suvning ingichka ekvatorial tasmasi bo'lgan "slushball" bo'ladimi, hali ham javobsiz qolmoqda.

Snowball-Earth epizodlari to'satdan oldin sodir bo'lgan deb taxmin qilinadi nurlanish nomi bilan tanilgan ko'p hujayrali bioformalarning Kembriya portlashi. Eng so'nggi qor to'pi epizodi ko'p hujayralilik rivojlanishiga turtki bo'lishi mumkin. Boshqa, ancha oldinroq va uzoqroq qorli epizod, Huron muzligi 2400 dan 2100 Myagacha sodir bo'lgan bo'lar edi, atmosferada birinchi bo'lib kislorod paydo bo'lishi "Ajoyib oksigenatsiya hodisasi ".

Tarix

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Qadimgi muzlik tog'lari uchun dalillar

Global muzlik g'oyasi paydo bo'lishidan ancha oldin, bir qator kashfiyotlar qadimiy prekambriyadagi muzliklarga oid dalillarni to'play boshladi. Ushbu kashfiyotlardan birinchisi 1871 yilda muzliklarda qayta ishlangan qadimiy materialni topgan J. Tomson tomonidan nashr etilgan (tillit ) ichida Islay, Shotlandiya. Xuddi shunday topilmalar Avstraliyada (1884) va Hindistonda (1887) kuzatilgan. To'rtinchi va juda tushunarli topilma "nomi bilan tanilganReyxning Morenasi "tomonidan xabar qilingan Xans Reush 1891 yilda Norvegiyaning shimoliy qismida. Ko'pgina boshqa topilmalar ortidan paydo bo'ldi, ammo ularning tushunilishiga rad etish to'sqinlik qildi kontinental drift.[5]

Global muzlik taklif qilingan

Janob Duglas Mawson (1882–1958), avstraliyalik geolog va Antarktida tadqiqotchisi, kariyerasining katta qismini Neoproterozoy stratigrafiya U Janubiy Avstraliyada, u qalin va keng muzlik cho'kindi jinslarini aniqlagan va karerasining oxirida global muzlik ehtimoli haqida taxmin qilgan.[6]

Mawsonning global muzlik haqidagi g'oyalari, ammo Avstraliyaning geografik joylashuvi va past kenglikdagi muzlik yotqiziqlari topilgan boshqa qit'alarning fikri vaqt o'tishi bilan doimiy bo'lib qoldi, degan yanglish taxminlarga asoslangan edi. Ning rivojlanishi bilan kontinental drift gipoteza va nihoyat plitalar tektonik nazariyasiga ko'ra, glatsiogen cho'kindilarni osonroq tushuntirish mumkin edi - ular materiklar yuqori kengliklarda bo'lgan davrda yotqizilgan.

1964 yilda global miqyosdagi muzlik g'oyasi qachon paydo bo'ldi V. Brayan Xarland taqdim etgan qog'ozini nashr etdi paleomagnetik ushbu muzlikni ko'rsatadigan ma'lumotlar tillitlar yilda Svalbard va Grenlandiya tropik kengliklarda yotqizilgan.[7] Ushbu paleomagnit ma'lumotlardan va muzlik cho'kindilarining odatda tropik va mo''tadil kengliklar bilan bog'liq bo'lgan jinslarning ketma-ketligini to'xtatganligi haqidagi sedimentologik dalillardan kelib chiqib, u muzlik davri bu shunchalik haddan tashqari ediki, tropik mintaqada dengiz muzlik jinslari cho'kindi.

1960-yillarda, Mixail Budyko, sovet klimatologi, muz qoplamining global ta'sirini o'rganish uchun oddiy energiya-muvozanat iqlim modelini ishlab chiqdi iqlim. Budyko ushbu modeldan foydalanib, agar muz qatlamlari qutbli hududlardan etarlicha uzoqlashsa, aks ettirish qobiliyati oshgan joyda (albedo ) muzning yanada sovishi va ko'proq muzning paydo bo'lishiga olib keldi, toki butun Yer muz bilan qoplanguncha va yangi muz bilan qoplangan muvozanatda barqarorlashdi.[8]

Budykoning modeli bu muz-albedo barqarorligi sodir bo'lishi mumkinligini ko'rsatgan bo'lsa-da, u aslida hech qachon bunday bo'lmagan degan xulosaga keldi, chunki uning modeli bunday teskari aloqa tizimidan qochib qutulishning iloji yo'q edi. 1971 yilda amerikalik fizik Aron Faegre shunga o'xshash energiya balansi modeli uchta barqaror global iqlimni bashorat qilganini, ulardan biri qor to'pi er ekanligini ko'rsatdi.[9]

Ushbu model taqdim etildi Edvard Norton Lorenz bir iqlimdan ikkinchisiga, shu jumladan qor to'pli erga katta sakrash bo'lishi mumkinligini ko'rsatadigan beparvolik tushunchasi.

"Qor to'pi Yer" atamasi tomonidan kiritilgan Jozef Kirshvink biologiyasiga oid 1992 yilda chop etilgan qisqa maqolada Proterozoy eon.[10] Ushbu ishning asosiy hissalari quyidagilar edi: (1) mavjudligini tan olish bantli temir shakllanishlari bunday global muzlik epizodiga va (2) butunlay muz bilan qoplangan Erdan qochish mexanizmini joriy etishga, xususan, CO to'planishiga mos keladi.2 vulkanik gazdan ultra-issiqxona effekti.

Franklin Van Xouten tomonidan ko'llar sathi ko'tarilib, pasayib boradigan izchil geologik naqshni kashf etishi endi "Van Xouten tsikli" deb nomlanmoqda. Uning fosfor konlari va bantli temir shakllanishlari cho'kindi jinslarda uni "qor to'pi Yer" gipotezasining dastlabki tarafdoriga aylantirdi, bu sayyora yuzasi 650 million yildan ko'proq vaqt oldin muzlab qoldi degan taxminni ilgari surdi.[11]

Kardan keyin Yer shari tushunchasiga qiziqish keskin ortdi Pol F. Xofman va uning hamkasblari Kirshvink g'oyalarini neoproterozoyik cho'kindi jinslarning ketma-ketligi Namibiya va jurnaldagi gipotezani batafsil ishlab chiqdi Ilm-fan kabi kuzatuvlarni o'z ichiga olgan holda 1998 yilda karbonatlar.[12]

2010 yilda Frensis Makdonald buni tasdiqladi Rodiniya davomida ekvatorial kenglikda bo'lgan Kriogen dengiz sathida yoki undan pastda muzli muz bo'lgan davr va shu bilan bog'liq Sturt muzligi global edi.[13]

Dalillar

Qor to'pi Yer gipotezasi dastlab tropik kengliklarda muzliklarning aniq ko'rinishini geologik dalillarni tushuntirish uchun ishlab chiqilgan.[14] Modellashtirish bo'yicha, an muz-albedo haqida fikr-mulohazalar muzliklar 25 ° gacha tarqalgandan so'ng, muzlik muzining ekvatorga tez o'tishiga olib keladi[15] 30 ° gacha[16] ekvatorning Shuning uchun muzlik yotqiziqlari tarkibida tropiklar global muz qoplamini taklif qiladi.

Binobarin, nazariyaning haqiqiyligini baholash uchun muzning tropik mintaqalarga etib borganligiga ishonch hosil qilgan dalillarning ishonchliligi va ahamiyatini anglash juda muhimdir. Ushbu dalillar uchta narsani isbotlashi kerak:

  1. to'shakda faqat muzlik faoliyati natijasida hosil bo'lishi mumkin bo'lgan cho'kindi tuzilmalar mavjud;
  2. yotoq yotqizilganida tropik mintaqada yotar edi.
  3. muzliklar bir vaqtning o'zida turli xil global joylarda faol bo'lganligi va o'sha yoshdagi boshqa konlar mavjud emasligi.

Ushbu so'nggi fikrni isbotlash juda qiyin. Oldin Ediakaran, biostratigrafik odatda jinslarni o'zaro bog'lash uchun ishlatiladigan markerlar mavjud emas; shuning uchun Yer sharining turli joylaridagi toshlar bir vaqtning o'zida yotqizilganligini isbotlashning imkoni yo'q. Amalga oshiriladigan eng yaxshi narsa - bu jinslarning yoshini taxmin qilish radiometrik kamdan-kam hollarda million yildan ko'proq vaqtga to'g'ri keladigan usullar.[17]

Dastlabki ikkita fikr ko'pincha har bir holat bo'yicha tortishuvlarga sabab bo'ladi. Ko'plab muzlik xususiyatlarini muzlik bo'lmagan usullar bilan yaratish mumkin va hanuzgacha bo'lgan quruqlik massivlarining taxminiy kengliklarini taxmin qilish 200 million yil oldin qiyinchiliklardan xalos bo'lish mumkin.[18]

Paleomagnetizm

Qor to'pi Yer gipotezasi avval ekvator yaqinidagi muzlik yotqiziqlari deb hisoblangan narsalarni tushuntirish uchun ilgari surilgan. Tektonik plitalar vaqt o'tishi bilan asta-sekin harakatlanayotganligi sababli, ularning uzoq vaqt tarixidagi ma'lum bir nuqtada o'z o'rnini aniqlash oson emas. Taniqli quruqlik bir-biriga qanday mos tushishi mumkinligi haqidagi fikrlardan tashqari, tosh yotqizilgan kenglik paleomagnetizm tomonidan cheklanishi mumkin.

Qachon cho'kindi jinslar ularning tarkibidagi magnit minerallar o'zlarini Yerning magnit maydoni. Buni aniq o'lchash orqali paleoomagnetizm, buni taxmin qilish mumkin kenglik (lekin emas uzunlik ) bu erda matritsa hosil bo'lgan. Paleomagnitik o'lchovlar shuni ko'rsatdiki, muzlikdan kelib chiqqan ba'zi cho'kmalar Neoproterozoy toshlar ekvatordan 10 darajagacha saqlangan,[19] garchi ushbu qayta qurishning to'g'riligi savol ostida bo'lsa.[17] Ko'rinishidan muzlik cho'kindilarining bu paleomagnit joylashuvi (masalan tomchilar ) cho'kindi jinslar yotqizilgan paytda muzliklarning tropik kengliklarda quruqlikdan dengiz sathigacha cho'zilganligini taxmin qilish uchun qabul qilingan. Bu global muzliklarni anglatadimi yoki mahalliy, ehtimol quruqlik bilan bog'liq bo'lgan muzlik rejimlarining mavjudligini anglatadimi, aniq emas.[20] Boshqalar hattoki ko'pgina ma'lumotlar ekvatordan 25 ° gacha bo'lgan muzlik qatlamlarini cheklamaydi deb taxmin qilishgan.[21]

Skeptiklar, agar paleomagnitik ma'lumotlar buzilishi mumkin, agar Yerning qadimgi magnit maydoni hozirgi zamonnikidan ancha farq qilsa edi. Ning sovutish tezligiga qarab Yerning yadrosi, proterozoy davrida, magnit maydon oddiy taxmin qilmadi dipolyar shimoliy va janubiy magnit qutblari sayyoramiz o'qi bilan taqqoslaganda, ular hozirgi kabi. Buning o'rniga, issiqroq yadro kuchliroq aylanib, 4, 8 yoki undan ortiq qutblarni keltirib chiqargan bo'lishi mumkin. Paleomagnitik ma'lumotlarni qayta izohlash kerak bo'ladi, chunki cho'kindi minerallar Shimoliy qutbga emas, balki "G'arbiy qutbga" ishora qilishi mumkin edi. Shu bilan bir qatorda, Yerning dipolyar maydonini qutblar ekvatorga yaqinlashadigan qilib yo'naltirish mumkin edi. Ushbu gipoteza Ediakaran paleomagnitik yozuvidan kelib chiqadigan magnit qutblarning favqulodda tezkor harakatini tushuntirish uchun yaratilgan; shimoliy qutbning taxmin qilingan harakati Gaskiers muzligi bilan bir vaqtda sodir bo'ladi.[22]

Paleomagnitik ma'lumotlarga ishonishning yana bir kuchsizligi bu qayd etilgan magnit signalning aslligini yoki keyinchalik faoliyat bilan qayta tiklanganligini aniqlashda qiyinchilik. Masalan, tog 'qurilishi orogeniya metamorfik reaktsiyalarning yon mahsuloti sifatida issiq suvni chiqaradi; bu suv minglab kilometr uzoqlikdagi toshlarga aylanib, magnit imzolarini qayta tiklay oladi. Bu bir necha million yildan oshgan jinslarning haqiqiyligini aniq mineralogik kuzatuvlarsiz aniqlash qiyinlashtiradi.[15] Bundan tashqari, paleomagnitik qutblarning taxminiy pozitsiyalarini qayta ko'rib chiqishni talab qilishi mumkin bo'lgan keng ko'lamli remagnetizatsiya hodisalari sodir bo'lganligi to'g'risida yana bir dalillar to'planmoqda.[23][24]

Hozirda faqat bitta kon, ya'ni Avstraliyaning Elatina koni mavjud bo'lib, u shubhasiz past kengliklarda joylashtirilgan; uning yotqizish sanasi juda cheklangan va signal namoyish etiladigan darajada o'ziga xosdir.[25]

Past kenglikdagi muzlik yotqiziqlari

Diamiktit ning Neoproterozoy Pocatello Formation, "qor to'pi Yer" tipidagi kon
Elatina Fm diamiktit quyida Ediakaran GSSP sayt Flinders NP oralig'ida, Janubiy Avstraliya. 1 dollarlik tanga o'lchov uchun.

Muzliklar tomonidan yotqizilgan cho'kindi jinslar ularni aniqlashga imkon beradigan o'ziga xos xususiyatlarga ega. Paydo bo'lishidan ancha oldin qartopi Yer faraz ko'p Neoproterozoy cho'kindilar muzlik kelib chiqishi deb talqin qilingan edi, shu jumladan ba'zi birlari cho'kindi chog'ida tropik kengliklarda. Shuni esda tutish kerakki, an'anaviy ravishda muzliklar bilan bog'liq bo'lgan ko'plab cho'kindi xususiyatlar boshqa yo'llar bilan ham shakllanishi mumkin.[26] Shunday qilib, qor to'pi Er uchun juda ko'p uchraydigan hodisalarning muzlik kelib chiqishi bahslashmoqda.[17]2007 yilga kelib, bitta "juda ishonchli" mavjud edi - bu hali ham qiyin[17]- tropik mintaqani aniqlaydigan aniq nuqta tillitlar,[19] bu ekvatorial muz qoplamini biroz tahmin qiladi. Biroq, tropik mintaqalarda dengiz sathining muzlashi dalillari Sturtian to'planib bormoqda.[27][28]Cho'kindining muzlikdan kelib chiqishi mumkin bo'lgan dalillarga quyidagilar kiradi:

  • Dropstones (dengiz cho'kindilariga tushgan toshlar), ular muzliklar yoki boshqa hodisalar bilan birikishi mumkin.[29]
  • Varves (periglasial ko'llarda yillik cho'kindi qatlamlari), ular yuqori haroratlarda hosil bo'lishi mumkin.[30]
  • Muzlik sohalari (tog 'jinslari bilan qirib tashlangan ko'milgan toshlar tomonidan hosil qilingan): vaqti-vaqti bilan o'xshash chiziqlar toshqinlar yoki tektonik harakatlar.[31]
  • Diamiktitlar (yomon tartiblangan konglomeratlar). Dastlab muzlik deb ta'riflangan qadar, aksariyati aslida tomonidan shakllangan chiqindilar oqadi.[17]

Ochiq suv konlari

Ko'rinib turibdiki, qor to'pi davrida hosil bo'lgan ba'zi yotqiziqlar faqat faol gidrologik tsikl mavjud bo'lganda paydo bo'lishi mumkin edi. Qalinligi 5500 metrgacha bo'lgan muzlik yotqiziqlari, muzlik bo'lmagan cho'kindilarning kichik (metr) polosalari bilan ajratilgan bo'lib, muzliklar o'n millionlab yillar davomida qayta-qayta erigan va qayta shakllanganligini namoyish etadi; qattiq okeanlar bunday cho'ktirishga yo'l qo'ymaydi.[32] Bu ko'rib chiqilmoqda[kim tomonidan? ] mumkin muz oqimlari kabi ko'rinishda Antarktida Bugungi kunda ushbu ketma-ketliklar paydo bo'lishi mumkin edi. Bundan tashqari, faqat ochiq suvda hosil bo'lishi mumkin bo'lgan cho'kindi xususiyatlar (masalan: to'lqin shaklidagi to'lqinlar, uzoq sayohat muzli qoldiqlar va fotosintez faolligining ko'rsatkichlari) qor to'pi-Yer davridan kelib chiqqan cho'kindi jinslarda uchraydi. Ular "vohalar" ni anglatishi mumkin erigan suv butunlay muzlagan Yerda,[33] kompyuterni modellashtirish shuni ko'rsatadiki, okeanning katta maydonlari muzsiz qolgan bo'lishi kerak; energiya balansi va umumiy aylanish modellari jihatidan "qattiq" qor to'pi ishonchli emasligini ta'kidlab.[34]

Uglerod izotoplari nisbati

Ikkita barqaror izotoplar uglerod dengiz suvi: uglerod-12 (12C) va nodir uglerod-13 (13C), bu uglerod atomlarining taxminan 1,109 foizini tashkil qiladi.

Biokimyoviy jarayonlar, ulardan fotosintez bittasi, zajigalkani afzalroq qo'shishga moyil 12S izotopi. Shunday qilib, okeanda yashovchi fotosintezatorlar protistlar va suv o'tlari, juda ozayib ketishga moyil 13C, boshlang'ich tarkibidagi mo'l-ko'llikka nisbatan vulkanik Yer uglerodining manbalari. Shuning uchun fotosintezli hayotga ega bo'lgan okean quyi darajaga ega bo'ladi 13C /12Organik qoldiqlarda C nisbati va tegishli okean suvida yuqori nisbat. Litifikatsiyalangan cho'kindilarning organik tarkibiy qismi juda oz bo'lib qoladi, ammo o'lchov bilan tugaydi 13S

Yerning qor to'pi epizodi paytida nisbati bo'yicha tezkor va o'ta salbiy ekskursiyalar mavjud 13C dan 12S[35] Vaqtni yaqindan tahlil qilish 13Dunyo bo'ylab yotqiziqlardagi C 'boshoqlar' Neoproterozoyning oxirida sodir bo'lgan to'rtta, ehtimol beshta muzlik hodisalarini tan olishga imkon beradi.[36]

Bantli temir shakllari

2,1 milliard yillik tosh qora lentali temir tosh bilan

Bantli temir shakllari (BIF) qatlamli cho'kindi jinslardir temir oksidi va temir kambag'al chert. Kislorod ishtirokida, temir tabiiy ravishda zanglaydi va suvda erimaydi. Tarmoqli temir shakllanishlar odatda juda qadimgi va ularning cho'kishi ko'pincha Yer atmosferasining oksidlanishiga bog'liq Paleoproterozoy okeandagi eritilgan temir fotosentetik ravishda hosil bo'lgan kislorod bilan aloqa qilganda va temir oksidi sifatida cho'kib ketganida.

Bantlar ishlab chiqarilgan uchish nuqtasi o'rtasida anoksik va kislorod bilan ta'minlangan okean. Chunki bugungi atmosfera shunday kislorod - boy (hajmi bo'yicha qariyb 21%) va okeanlar bilan aloqa qilganda, temirli oksidi to'plangan qatlamni yotqizish uchun etarli miqdorda to'plash mumkin emas. Paleoproterozoydan keyin (1,8 milliard yil oldin) yotqizilgan yagona keng temir shakllanishlar Kriogen muzlik konlari.

Bunday temirga boy jinslar yotqizilishi uchun okeanda anoksiya bo'lishi kerak edi, shuning uchun juda ko'p erigan temir ( temir oksidi ) oksidlovchi bilan uchrashguncha to'planib qolishi mumkin edi, chunki uni cho'ktirishi mumkin temir oksid. Okean anoksik bo'lishi uchun u kislorodli atmosfera bilan cheklangan gaz almashinuviga ega bo'lishi kerak. Gipoteza tarafdorlarining ta'kidlashicha, cho'kindi yozuvlarda BIF ning yana paydo bo'lishi dengiz muzlari bilan yopilgan okeandagi kislorod miqdori cheklanganligi natijasidir,[10] muxoliflar esa BIF konlarining kamligi ularning ichki dengizlarda hosil bo'lganligini ko'rsatishi mumkin deb taxmin qilishmoqda.

Bunday ko'llar okeanlardan ajratilgan holda, bugungi suv singari turg'un va anoksik bo'lishi mumkin edi Qora dengiz; etarli miqdordagi temir BIF hosil bo'lishi uchun zarur shart-sharoitlarni ta'minlay oladi.[17] BIF-lar muzlikning oxirini belgilagan deb taxmin qilishning yana bir qiyin tomoni shundaki, ular muzlik cho'kindi jinslari bilan aralashgan holda topilgan.[20] BIF-lar ham ajoyib tarzda yo'q Marino muzligi.[iqtibos kerak ]

Kapak karbonat jinslari

Hozirgi muzlik

Ustki qismi atrofida Neoproterozoy muzlik konlari odatda kimyoviy cho'kindi cho'kindi jinsga keskin o'tib boradi ohaktosh yoki dolomit metrdan o'n metrgacha qalinligi.[37] Ushbu qopqoqli karbonatlar ba'zida boshqa karbonat jinslari bo'lmagan cho'kindi ketma-ketlikda uchraydi va bu ularning cho'kishi chuqur aberratsiya natijasida okean kimyosi.[38]

To'ldirishda vulkanlar muhim rol o'ynagan bo'lishi mumkin CO
2, ehtimol global muzlik davri tugaydi Kriogen Davr.

Bular karbonatlar g'ayrioddiy kimyoviy tarkibga ega, shuningdek, ko'pincha katta to'lqinlar deb talqin qilinadigan g'alati cho'kindi tuzilmalar.[39]Bunday cho'kindi jinslarning paydo bo'lishiga musbat zaryadlanganlarning katta oqimi sabab bo'lishi mumkin ionlari, Yer sharidagi qor voqeasi ortidan haddan tashqari issiqxonada tez ob-havo natijasida hosil bo'ladigan narsa. The δ13C qopqoq karbonatlarning izotopik imzosi −5 near ga yaqin, mantiya qiymatiga mos keladi - bunday past qiymat hayotning yo'qligini bildirish uchun odatda olinadi / olinishi mumkin, chunki fotosintez odatda qiymatni oshirish uchun harakat qiladi; Shu bilan bir qatorda metan qatlamlarining chiqishi uni yuqori qiymatdan tushirishi va fotosintez ta'sirini muvozanatlashtirishi mumkin edi.

Qopqoq karbonatlarning hosil bo'lishining aniq mexanizmi aniq emas, ammo eng ko'p keltirilgan izoh shuni ko'rsatadiki, Yer sharining erishi paytida suv mo'l-ko'l eriydi CO
2 dan atmosfera shakllantirmoq karbonat kislota kabi tushishi mumkin edi kislotali yomg'ir. Bu ob-havo ta'sirida bo'ladi silikat va karbonat tosh (shu jumladan, shiddat bilan hujumga uchragan muzlik qoldiqlari), ko'p miqdordagi chiqindilar kaltsiy Okeanga yuvilganda karbonatli cho'kindi jinslarning o'ziga xos teksturali qatlamlari hosil bo'ladi. Bunday abiotik "qopqoq karbonat "muzlik ustida cho'kindi jinslarni topish mumkin.

Biroq, muzlik kelib chiqishini qopqoq karbonatlarga belgilashda ba'zi muammolar mavjud. Birinchidan, atmosferadagi yuqori karbonat angidrid konsentratsiyasi okeanlarning kislotali bo'lishiga olib keladi va tarkibidagi har qanday karbonatlarni eritib yuboradi - bu qopqoq karbonatlarning cho'ktirilishi bilan keskin farq qiladi. Bundan tashqari, ba'zi bir qopqoq karbonatlarning qalinligi nisbatan tezroq deglyatsiyada hosil bo'lishi mumkin bo'lgan darajadan ancha yuqori. Shunga o'xshash vaqtda aniq muzlik kelib chiqishining ko'plab ketma-ketliklari ustidagi kepkali karbonatlarning etishmasligi va shunga o'xshash karbonatlarning taklif qilingan muzlik kelib chiqishi ketma-ketliklari ichida paydo bo'lishi tufayli sabab yanada zaiflashadi.[17] Ishlab chiqargan bo'lishi mumkin bo'lgan muqobil mexanizm Doushantuo hech bo'lmaganda qopqoq karbonat, metanning tez va keng tarqalishi. Bu juda past - −48 ‰ gacha -δ13C qiymatlar - shuningdek, cho'kindilar orqali gaz oqimi natijasida hosil bo'lgan ko'rinadigan g'ayrioddiy cho'kindi xususiyatlar.[40]

O'zgaruvchan kislota

Elementning izotoplari bor deb taklif qilaman pH oldin va keyin okeanlarning keskin pasayishi Marino muzlik.[41]Bu to'planishni ko'rsatishi mumkin karbonat angidrid atmosferada, ularning ba'zilari hosil bo'lish uchun okeanlarda eriydi karbonat kislota. Borning xilma-xilligi haddan tashqari darajadagi dalil bo'lishi mumkin Iqlim o'zgarishi, ular global muzlik degani emas.

Kosmik chang

Yer yuzasi elementda juda ozayib ketgan iridiy birinchi navbatda Yerning yadrosida joylashgan. Sirtdagi elementning yagona muhim manbai bu kosmik zarralar Yerga etib boradi. Yer yuzida qor to'pi paytida iridiy muz qatlamlarida to'planib borar edi va muz eritilganda hosil bo'lgan cho'kindi qatlami iridiyga boy bo'ladi. An iridiy anomaliyasi qopqoq karbonat shakllanishi asosida topilgan va muzlik epizodi kamida 3 million yil davom etganligini taxmin qilish uchun ishlatilgan,[42] ammo bu albatta a degani emas global muzlik darajasi; haqiqatan ham shunga o'xshash anomaliyani katta ta'sir bilan izohlash mumkin edi meteorit.[43]

Iqlimning tsiklik o'zgarishi

Mobil aloqaning nisbati kationlar davomida tuproqda qolganlarga kimyoviy ob-havo (o'zgarishning kimyoviy ko'rsatkichi) kimyoviy buzilish muzlik ketma-ketligi ichida tsiklik usulda o'zgarib turishi, muzlararo davrlarda ko'payib, sovuq va quruq muzlik davrlarida pasayib borishi ko'rsatilgan.[44] Ushbu naqsh, agar voqealarning haqiqiy aksi bo'lsa, "qor to'pi erlari" ga o'xshashligini yanada kuchaytiradi Pleystotsen muzlik davri to'liq muzlatilgan Yerga qaraganda tsikllar.

Bundan tashqari, muzlikning cho'kindi jinslari Port Askaig Tillit shakllanishi Shotlandiyada muzli va sayoz dengiz cho'kindilarining qatlamlari aniq ko'rsatilgan.[45] Ushbu konlarning ahamiyati ularning tarixiga juda bog'liq. Muzlik cho'kindilarini sanash qiyin va Portaskaig guruhiga eng yaqin tarixiy yotoq, qiziqish uyg'otadigan qatlamlardan 8 km balandlikda joylashgan. Uning 600 mln. Yilgacha bo'lganligi, ko'rpa-to'shaklarning Sturtiya muzligi bilan taxminiy ravishda bog'liqligini anglatadi, ammo ular erning qor to'pi oldinga siljishini yoki orqaga qaytishini anglatishi mumkin.

Mexanizmlar

Dunyoda qor to'pi bo'lgan davrdagi bitta kompyuter simulyatsiyasi[46]

Yer yuzidagi qorli hodisani boshlash ba'zi bir dastlabki sovutish mexanizmlarini o'z ichiga oladi, bu esa Yerning qor va muz bilan qoplanishining ko'payishiga olib keladi. Yerning qor va muz bilan qoplanishining ko'payishi o'z navbatida Yerning ko'payishiga olib keladi albedo, natijada nima bo'ladi ijobiy fikr sovutish uchun. Agar etarli miqdordagi qor va muz to'planib qolsa, sovib ketishi mumkin. Ushbu ijobiy teskari aloqani ekvatorial kontinental taqsimot osonlashtiradi, bu esa ekvatorga yaqin mintaqalarda muz to'planishiga imkon beradi. quyosh radiatsiyasi to'g'ridan-to'g'ri.

Ko'plab qo'zg'atuvchi mexanizmlar Yerning qor to'planishi boshlanishini hisobga olishi mumkin, masalan, a ning otilishi Supervulkan, ning atmosfera kontsentratsiyasining pasayishi issiqxona gazlari kabi metan va / yoki karbonat angidrid, o'zgarishlar Quyosh energiyasi chiqishi, yoki bezovtalanishi Yerning orbitasi. Qanday qo'zg'atuvchidan qat'i nazar, dastlabki sovutish natijasida Yer yuzining muz va qor bilan qoplanishi ko'payadi va qo'shimcha muz va qor ko'proq Quyosh energiyasini kosmosga qaytaradi, Yerni yanada sovitadi va Yer yuzasining maydonini yanada oshiradi muz va qor. Ushbu ijobiy qayta aloqa davri oxir-oqibat muzlatilgan bo'lishi mumkin ekvator zamonaviy kabi sovuq Antarktida.

Global isish millionlab yillar davomida atmosferada karbonat angidridning katta miqdordagi to'planishi bilan bog'liq bo'lib, asosan vulkanik faollik bilan ajralib turadi, bu Erni qor uchun eritish uchun taklif qilingan omil. Erish uchun ijobiy teskari aloqa tufayli, er yuzining aksariyat qismini qoplagan qor va muzning erishi ming yillikni talab qiladi.[iqtibos kerak ]

Kontinental taqsimot

Materiklarning tropik taqsimoti, ehtimol qarama-qarshi intuitiv ravishda, qartopi Yerni boshlashiga imkon berish uchun zarurdir.[47]Birinchidan, tropik qit'alar ochiq okeanga qaraganda ko'proq aks ettiradi va shu sababli Quyosh issiqligini ozroq shimib oladi: Quyosh energiyasining Yerdagi eng ko'p yutilishi tropik okeanlarda sodir bo'ladi.[48]

Bundan tashqari, tropik qit'alarda ko'proq yog'ingarchilik bo'ladi, bu esa daryolarning oqimi va eroziyasini ko'payishiga olib keladi. silikat jinslar atmosferadan karbonat angidridni olib tashlaydigan ob-havoning reaktsiyalariga uchraydi. Ushbu reaktsiyalar umumiy shaklda davom etadi: Tosh hosil qiluvchi mineral + CO2 + H2O → kationlar + bikarbonat + SiO2. Bunday reaktsiyaga misol bo'lib, vollastonit:

CaSiO3 + 2CO2 + H2O → Ca2+ + SiO2 + 2HCO3−

Ozod qilindi kaltsiy kationlar erigan bilan reaksiyaga kirishadi bikarbonat hosil qilish uchun okeanda kaltsiy karbonat kimyoviy cho'kma sifatida cho'kindi jinslar. Ushbu transferlar karbonat angidrid, havodan issiqxonaga geosfera, va geologik vaqt o'lchovlari bo'yicha barqaror holatda, chiqadigan karbonat angidridni ofset qiladi vulqonlar atmosferaga.

2003 yildan boshlab neoproterozoy davrida aniq kontinental taqsimotni o'rnatish qiyin edi, chunki tahlil uchun mos cho'kindilar juda kam edi.[49] Ba'zi rekonstruksiyalar qutbli qit'alarga to'g'ri keladi - bu boshqa barcha muzliklarning o'ziga xos xususiyati bo'lib, muzning yadrolashi mumkin bo'lgan nuqtani beradi. Okean sirkulyasiyasining o'zgarishi, keyinchalik Yer sharining qo'zg'alishini ta'minlagan bo'lishi mumkin.[50]

Neoproterozoy qor to'pi boshlanishiga sabab bo'lishi mumkin bo'lgan qo'shimcha omillarga atmosferada erkin kislorodning kiritilishi kiradi, ular reaksiyaga kirishish uchun etarli miqdorga etgan bo'lishi mumkin. atmosferadagi metan, uni karbonat angidridga oksidlovchi, ancha kuchsizroq issiqxona gazi,[51] Neoproterozoyda 6 foiz kam radiatsiya chiqaradigan Quyosh va shuning uchun zaifroq Quyosh.[17]

Odatda, tabiiy iqlim o'zgarishi va kirib kelayotgan quyosh radiatsiyasining o'zgarishi tufayli Yer sovuqlashganda, sovutish bu ob-havo reaktsiyalarini sekinlashtiradi. Natijada atmosferadan kam karbonat angidrid chiqariladi va bu issiqxona gazlari to'planib borishi bilan Yer isiydi - bu 'salbiy teskari aloqa 'jarayoni sovutish hajmini cheklaydi. Davomida Kriogen davrda, ammo Yer qit'alari hammasi edi tropik kengliklari, bu mo''tadil jarayonni unchalik samarasiz qildi, chunki Yer soviganida ham quruqlikda yuqori ob-havo darajasi davom etdi. Bu muzning qutbli hududlardan tashqariga chiqishiga imkon beradi. Muz ekvatordan 30 ° gacha ko'tarilgach,[52] ijobiy teskari aloqa shunday bo'lishi mumkinki, aks ettiruvchi (albedo ) muzning yanada sovishi va ko'proq muz hosil bo'lishiga olib keldi, toki butun Yer muz bilan qoplanguncha.

Qutb qit'alari, past stavkalari tufayli bug'lanish, atmosferada karbonat angidrid miqdorini cheklab qo'yadigan uglerodni cho'ktirishga imkon berish uchun juda quruq. uglerod aylanishi. Nisbati asta-sekin o'sishi izotop uglerod-13 uglerod-12 ga nisbatan cho'kindilarda "global" muzlik paydo bo'lishidan oldin buni ko'rsatadi CO
2 Qor to'pi oldidan tortib olinadigan Yerlar sekin va uzluksiz jarayon edi.[53]

Kartopu Yerlarning boshlanishi har doim $ Omega $ keskin pasayishi bilan ajralib turadi13Cho'kindilarning C qiymati,[54] sovuq harorat va muz bilan qoplangan okeanlar natijasida biologik mahsuldorlikning qulashi bilan bog'liq bo'lishi mumkin bo'lgan belgi.

2016 yil yanvar oyida Gernon va boshq. ning parchalanishini o'z ichiga olgan "sayoz tizma gipoteza" ni taklif qildi superkontinent Rodiniya, portlashi va tez o'zgarishini bog'lab turadi gialoklastitlar sayoz tizmalar bo'ylab ishqoriylikning ko'payishiga qadar qalin muz qatlami bo'lgan okeanda. Gernon va boshq. muzlash jarayonida ishqoriylikning ko'payishi Yer sharidagi hodisalar natijasida hosil bo'lgan qopqoq karbonatlarning qalinligini tushuntirish uchun etarli ekanligini namoyish etdi.[55]

Muzlatilgan davrda

Global muz qatlamlari ko'p hujayrali hayot evolyutsiyasi uchun zarur bo'lgan to'siqni yaratgan bo'lishi mumkin.[3]

Jahon harorati shunchalik pasayib ketdiki, ekvator hozirgi zamondek sovuq edi Antarktida.[56] Ushbu past harorat koinotga kiruvchi quyosh energiyasini aks ettiruvchi muz qatlamlarining yuqori albedosi yordamida saqlanib turdi. Atmosferadan suv bug'ining muzlashi oqibatida issiqlikni saqlaydigan bulutlarning etishmasligi bu ta'sirni kuchaytirdi.

Global muzlikdan chiqib ketish

The karbonat angidrid Erni eritish uchun zarur bo'lgan darajalar hozirgi darajadan 350 baravar, ya'ni atmosferaning 13 foizini tashkil etadi.[57] Yer deyarli butunlay muz bilan qoplanganligi sababli, atmosferadan karbonat angidridni ishqorli metal ionlari chiqarilishi natijasida chiqarib bo'lmaydi. kremniyli jinslar. 4 dan 30 million yilgacha CO
2 va metan, asosan tomonidan chiqarilgan vulqonlar shuningdek, muz ostida qolgan organik uglerodni gazga aylantiradigan mikroblar tomonidan ishlab chiqarilgan,[58] nihoyatda doimiy muzsiz er va suv bo'lagi paydo bo'lguncha tropikda er usti muzlari erishi uchun etarli darajada issiqxona effekti hosil qilish uchun to'planadi;[59] bu muzdan qoraygan bo'lar edi va shu bilan Quyoshdan ko'proq energiya oladi - "ijobiy fikr ".

Ning muhim konlarini barqarorlashtirish metan gidratlari past kenglikda qamalgan doimiy muzlik deglasatsiya va isish uchun qo'zg'atuvchi va / yoki kuchli ijobiy teskari aloqa vazifasini ham bajargan bo'lishi mumkin.[60]

Qit'alarda, erishi muzliklar katta miqdordagi muzlik konini bo'shatib yuboradi, bu esa yemirilishi va ob-havo. Okeanga etkazib beriladigan cho'kindilar kabi ozuqaviy moddalarga boy bo'ladi fosfor ko'pligi bilan birlashtirilgan CO
2 qo'zg'atadi a siyanobakteriyalar atmosferaning nisbatan tez reoksigenatsiyasini keltirib chiqaradigan populyatsiya portlashi va bu ko'tarilishga yordam bergan bo'lishi mumkin Ediakaran biota va keyingi Kembriya portlashi - ko'p hujayrali hayot shakllarini rivojlanishiga imkon beradigan yuqori kislorod kontsentratsiyasi. Garchi ijobiy fikr pastadir geologik qisqa tartibda muzni eritishi mumkin, ehtimol 1000 yildan kamroq vaqt ichida, atmosferadagi kislorodni to'ldirish va uning yo'q bo'lib ketishi CO
2 darajalar yanada oshishi kerak edi ming yillik.

Ehtimol, karbonat angidrid darajasi Yerning yana muzlashi uchun tushgan bo'lishi mumkin; ushbu tsikl qadar takrorlangan bo'lishi mumkin qit'alar siljigan edi ko'proq qutb kengliklariga.[61]

Yaqinda o'tkazilgan dalillar shuni ko'rsatadiki, okean harorati soviganida, natijada okeanlarning gazlarni eritish qobiliyati yuqori bo'lib, dengiz suvidagi uglerod miqdori karbonat angidridga tezroq oksidlanadi. Bu to'g'ridan-to'g'ri atmosferadagi karbonat angidrid gazining ko'payishiga, Yer yuzining issiqxonalarining iliqlashishiga va umuman qor to'pi holatining oldini olishga olib keladi.[62]

Million yillar davomida, kriyokonit muz ustida va ichida to'planib qolgan bo'lar edi. Psixrofil mikroorganizmlar, vulkanik kul va muzsiz joylardan olingan chang bir necha million kvadrat kilometrni egallagan muzga joylashadi. Muzlar eriy boshlagach, bu qatlamlar ko'rinadigan bo'lib, muzli yuzalarni quyuq rangga bo'yab, jarayonni tezlashtirishga yordam beradi.[63]

Quyoshdan keladigan ultrabinafsha nurlar vodorod peroksidni ham hosil qiladi (H2O2) suv molekulalariga urilganda. Odatda vodorod periksidi quyosh nurlari bilan parchalanadi, ammo ba'zilari muz ichida qolib ketgan bo'lar edi. Muzliklar eriy boshlaganida, u ham okeanda, ham atmosferada tarqalib, suv va kislorod molekulalariga bo'linib atmosfera kislorodining ko'payishiga olib kelgan bo'lar edi.[64]

Slushball Earth gipotezasi

Muzliklarning mavjudligi haqida tortishuvlarga yo'l qo'yilmasa ham, butun sayyoramiz muz bilan qoplangan degan fikr yanada munozarali bo'lib, ba'zi olimlar "shilimshiq Yer" ni yaratishga olib keladi, unda muzsiz yoki muzli suvlar bo'lagi qoladi atrofida ekvator, davom ettirishga imkon beradi gidrologik tsikl.

Ushbu gipoteza cho'kindi yozuvlarning faqat ochiq suv ostida yoki tez harakatlanayotgan muz ostida hosil bo'lishi mumkin bo'lgan ba'zi xususiyatlarini kuzatadigan olimlarga murojaat qiladi (bu erda muzsiz joyga o'tish kerak bo'ladi). Yaqinda o'tkazilgan tadqiqotlar geokimyoviy tsiklikliligini kuzatdi jinslar, "qor to'pi" davrlari shunga o'xshash iliq sehrlar bilan tinilganligini ko'rsatmoqda muzlik davri so'nggi Yer tarixidagi tsikllar. Dunyo bo'ylab qor to'pi bo'lgan kompyuter modellarini yaratishga urinishlar sayyorani boshqaradigan qonunlar va konstantalarda tub o'zgarishlarsiz global muz qoplamini joylashtirishga qiynaldi.

Kamroq haddan tashqari qor to'pi bo'lgan Yer gipotezasi muttasil rivojlanib boruvchi kontinental konfiguratsiyalarni va okean aylanishidagi o'zgarishlarni o'z ichiga oladi.[65] Sintez qilingan dalillarda "slushball Earth" ni ko'rsatadigan modellar ishlab chiqarilgan,[66] bu erda stratigrafik yozuv to'liq global muzliklarni postulatsiyalashga yo'l qo'ymaydi.[65] Kirschivinkning asl gipotezasi[10] qorli er yuzida iliq tropik ko'lmaklar bo'lishi kutilayotganini tan olgan edi.

Qor to'pi Yer gipotezasi muzlik va muzliklararo hodisalarning o'zgarishini ham, muzlik qatlamlari chegaralarining tebranishini ham tushuntirmaydi.[67]

Ilmiy nizo

Gipotezaga qarshi dalil "qor to'pi Yer" konlari paytida muz qatlamining o'zgarishi va erishi dalilidir. Bunday erish uchun dalillar muzlik tomchilari toshlaridan dalolat beradi,[32] iqlim davriyligining geokimyoviy dalillari,[44] va muzlik va sayoz dengiz cho'kindilari.[45] 13 ° N ga cheklangan Ummondan uzoqroq yozuv 712 yildan 545 million yilgacha bo'lgan davrni o'z ichiga oladi - bu vaqt oralig'ida Sturt va Marinoan muzliklar - va muzlik va muzsiz cho'kishni ko'rsatadi.[68]

Qor to'pi bo'lgan Yerni qayta tiklashda qiyinchiliklar bo'lgan global iqlim modellari. Ekvatorgacha muzlash uchun aralash qatlamli okeanlar bilan oddiy GKMlarni tayyorlash mumkin; to'liq dinamik okeanga ega bo'lgan yanada murakkab model (faqat ibtidoiy dengiz muz modeli bo'lsa ham) ekvatorga dengiz muzini hosil qila olmadi.[69] Bundan tashqari, darajalari CO
2 global muz qoplamini eritish uchun zarur bo'lgan 130,000 ppm deb hisoblangan,[57] bu asossiz katta deb hisoblanadi.[70]

Stronsiy izotopik ma'lumotlari muzlik paytida qor yog'ayotgan va silikatlarning ob-havoning yopilishining Yerdagi modellari va muzlikdan so'ng darhol tez sur'atlar bilan zid ekanligi aniqlandi. Shuning uchun metanning doimiy muzlikdan chiqarilishi dengiz qonunchiligi muzlikdan so'ng darhol o'lchovli uglerod ekskursiyasining manbai bo'lishi taklif qilingan.[71]

"Zipper rift" gipotezasi

Nick Eyles suggests that the Neoproterozoic Snowball Earth was in fact no different from any other glaciation in Earth's history, and that efforts to find a single cause are likely to end in failure.[17] The "Zipper rift" hypothesis proposes two pulses of continental "unzipping"—first, the breakup of the supercontinent Rodinia, forming the proto-Pacific Ocean; then the splitting of the continent Baltica dan Laurentiya, forming the proto-Atlantic—coincided with the glaciated periods.The associated tectonic uplift would form high plateaus, just as the Sharqiy Afrika Rift is responsible for high topography; this high ground could then host glaciers.

Banded iron formations have been taken as unavoidable evidence for global ice cover, since they require dissolved iron ions and anoxic waters to form; however, the limited extent of the Neoproterozoic banded iron deposits means that they may not have formed in frozen oceans, but instead in inland seas. Such seas can experience a wide range of chemistries; high rates of evaporation could concentrate iron ions, and a periodic lack of circulation could allow anoxic bottom water to form.

Continental rifting, with associated subsidence, tends to produce such landlocked water bodies. This rifting, and associated subsidence, would produce the space for the fast deposition of sediments, negating the need for an immense and rapid melting to raise the global sea levels.

High-obliquity hypothesis

A competing hypothesis to explain the presence of ice on the equatorial continents was that Earth's eksenel burilish was quite high, in the vicinity of 60°, which would place Earth's land in high "latitudes", although supporting evidence is scarce.[72] A less extreme possibility would be that it was merely Earth's magnit qutb that wandered to this inclination, as the magnetic readings which suggested ice-filled continents depend on the magnetic and rotational poles being relatively similar. In either of these two situations, the freeze would be limited to relatively small areas, as is the case today; severe changes to Earth's climate are not necessary.

Inertial interchange true polar wander

The evidence for low-latitude glacial deposits during the supposed snowball Earth episodes has been reinterpreted via the concept of inertial interchange true polar wander (IITPW).[73][74]This hypothesis, created to explain palaeomagnetic data, suggests that Earth's orientation relative to its axis of rotation shifted one or more times during the general time-frame attributed to snowball Earth. This could feasibly produce the same distribution of glacial deposits without requiring any of them to have been deposited at equatorial latitude.[75] While the physics behind the proposition is sound, the removal of one flawed data point from the original study rendered the application of the concept in these circumstances unwarranted.[76]

Several alternative explanations for the evidence have been proposed.

Survival of life through frozen periods

A qora chekuvchi, gidrotermal shamollatish turi

A tremendous glaciation would curtail photosynthetic life on Earth, thus depleting atmospheric oxygen, and thereby allowing non-oxidized iron-rich rocks to form.

Detractors argue that this kind of glaciation would have made life extinct entirely. However, microfossils such as stromatolitlar va onkolitlar prove that, in shallow marine environments at least, life did not suffer any perturbation. Instead life developed a trophic complexity and survived the cold period unscathed.[77] Proponents counter that it may have been possible for life to survive in these ways:

  • In reservoirs of anaerob and low-oxygen life powered by chemicals in deep oceanic gidrotermal teshiklar surviving in Earth's deep oceans and qobiq; lekin fotosintez would not have been possible there.
  • Under the ice layer, in xemolitotrofik (mineral-metabolizing) ekotizimlar theoretically resembling those in existence in modern glacier beds, high-alpine and Arctic talus permafrost, and basal glacial ice. This is especially plausible in areas of vulkanizm yoki geotermik faoliyat.[78]
  • In pockets of liquid water within and under the ice caps, similar to Vostok ko'li Antarktidada. In theory, this system may resemble mikrobial communities living in the perennially frozen lakes of the Antarctic dry valleys. Photosynthesis can occur under ice up to 100 m thick, and at the temperatures predicted by models equatorial sublimatsiya would prevent equatorial ice thickness from exceeding 10 m.[79]
  • As eggs and dormant cells and spores deep-frozen into ice during the most severe phases of the frozen period.
  • In small regions of open water in deep ocean regions preserving small quantities of life with access to light and CO
    2     for photosynthesizers (not multicellular plants, which did not yet exist) to generate traces of oxygen that were enough to sustain some oxygen-dependent organisms. This would happen even if the sea froze over completely, if small parts of the ice were thin enough to admit light. These small open water regions may have occurred in deep ocean regions far from the superkontinent Rodiniya or its remnants as it broke apart and drifted on the tektonik plitalar.
  • In layers of "dirty ice" on top of the ice sheet covering shallow seas below. Animals and mud from the sea would be frozen into the base of the ice and gradually concentrate on the top as the ice above evaporates. Small ponds of water would teem with life thanks to the flow of nutrients through the ice.[80] Such environments may have covered approximately 12 per cent of the global surface area.[81]
  • In small oases of liquid water, as would be found near geotermik qaynoq nuqtalar o'xshash Islandiya Bugun.[82]
  • Yilda nunatak mintaqalar tropiklar, where daytime tropical sun or volcanic heat heated bare rock sheltered from cold wind and made small temporary melt pools, which would freeze at sunset.
  • Oxygenated subglacial meltwater, along with iron-rich sediments dissolved in the glacial water, created a meltwater oxygen pump when it entered the ocean, where it provided eukaryotes with some oxygen, and both photosynthetic and chemosynthetic organisms with sufficient nutrients to support an ecosystem. The freshwater would also mix with the hypersaline seawater, which created areas less hostile to eukaryotic life than elsewhere in the ocean.[83]

However, organisms and ecosystems, as far as it can be determined by the fossil record, do not appear to have undergone the significant change that would be expected by a ommaviy qirilish. With the advent of more precise dating, a phytoplankton extinction event which had been associated with snowball Earth was shown to precede glaciations by 16 million years.[84] Even if life were to cling on in all the ecological refuges listed above, a whole-Earth glaciation would result in a biota with a noticeably different diversity and composition. This change in diversity and composition has not yet been observed[85]—in fact, the organisms which should be most susceptible to climatic variation emerge unscathed from the snowball Earth.[43] One rebuttal to this is the fact that in many of these places where an argument is made against a mass extinction caused by snowball earth, the Cryogenian fossil record is extraordinarily impoverished.[86]

Ta'siri

A snowball Earth has profound implications in the history of hayot Yerda. Ko'p bo'lsa ham refugia have been postulated, global ice cover would certainly have ravaged ekotizimlar dependent on sunlight. Geochemical evidence from rocks associated with low-latitude glacial deposits have been interpreted to show a crash in oceanic life during the glacials.

Because about half of the oceans' water was frozen solid as ice, the remaining water would be twice as salty as it is today, lowering its freezing point. When the ice sheet melted, it would cover the oceans with a layer of hot freshwater up to 2 kilometres thick. Only after the hot surface water mixed with the colder and deeper saltwater did the sea return to a warmer and less salty state.[87]

The melting of the ice may have presented many new opportunities for diversification, and may indeed have driven the rapid evolution which took place at the end of the Kriogen davr.

Effect on early evolution

Diksoniya kostalar, an Ediakaran organism of unknown affinity, with a quilted appearance

The Neoproterozoy was a time of remarkable diversification of multicellular organisms, including animals. Organism size and complexity increased considerably after the end of the snowball glaciations. This development of multicellular organisms may have been the result of increased evolutionary pressures resulting from multiple icehouse-hothouse cycles; in this sense, snowball Earth episodes may have "pumped" evolution. Alternatively, fluctuating nutrient levels and rising oxygen may have played a part. Another major glacial episode may have ended just a few million years before the Kembriya portlashi.

One hypothesis which has been gaining currency in recent years: that early snowball Earths did not so much ta'sir qilish the evolution of life on Earth as result from it. In fact the two hypotheses are not mutually exclusive. The idea is that Earth's life forms affect the global carbon cycle and so major evolutionary events alter the carbon cycle, redistributing carbon within various reservoirs within the biosphere system and in the process temporarily lowering the atmospheric (greenhouse) carbon reservoir until the revised biosphere system settled into a new state. The Snowball I episode (of the Huron muzligi 2.4 to 2.1 billion years) and Snowball II (of the Precambrian's Kriogen between 580 and 850 million years and which itself had a number of distinct episodes) are respectively thought to be caused by the evolution of kislorodli fotosintez and then the rise of more advanced multicellular animal life and life's colonization of the land.[88][89]

Effects on ocean circulation

Global ice cover, if it existed, may—in concert with geothermal heating—have led to a lively, well mixed ocean with great vertical convective circulation.[90]

Occurrence and timing

Neoproterozoy

There were three or four significant ice ages during the late Neoproterozoy. Ulardan Marino was the most significant, and the Sturtian glaciations were also truly widespread.[91] Even the leading snowball proponent Hoffman agrees that the 350 thousand-year-long[1] Gaskiers glaciation did not lead to global glaciation,[47] although it was probably as intense as the late Ordovician glaciation. Holati Kaigas "glaciation" or "cooling event" is currently unclear; some scientists do not recognise it as a glacial, others suspect that it may reflect poorly dated strata of Sturtian association, and others believe it may indeed be a third ice age.[92] It was certainly less significant than the Sturtian or Marinoan glaciations, and probably not global in extent. Emerging evidence suggests that the Earth underwent a number of glaciations during the Neoproterozoic, which would stand strongly at odds with the snowball hypothesis.[4]

Paleoproterozoy

Asosiy maqola: Huron muzligi

The snowball Earth hypothesis has been invoked to explain glacial deposits in the Huronian Supergroup of Canada, though the palaeomagnetic evidence that suggests ice sheets at low latitudes is contested.[93][94] The glacial sediments of the Makganyene formation of South Africa are slightly younger than the Huronian glacial deposits (~2.25 billion years old) and were deposited at tropical latitudes.[95] It has been proposed that rise of free oxygen that occurred during the Ajoyib oksigenatsiya hodisasi removed methane in the atmosphere through oxidation. Sifatida Quyosh was notably weaker at the time, Earth's climate may have relied on methane, a powerful greenhouse gas, to maintain surface temperatures above freezing.

In the absence of this methane greenhouse, temperatures plunged and a snowball event could have occurred.[94]

Karoo muzlik davri

Before the theory of continental drift, glacial deposits in Karbonli strata in tropical continental areas such as India and South America led to speculation that the Karoo muzlik davri glaciation reached into the tropics. However, a continental reconstruction shows that ice was in fact constrained to the polar parts of the superkontinent Gondvana.

Shuningdek qarang

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