ترى ماذا سيحدث حين تذوب الجبال الجليدية؟

[نجيب نون]

ينعقد في مدينة سان فرانسيكسو الأميركية اجتماع الاتحاد الجيوفيزيائي الأميركي، وهو أكبر تجمع سنوي عالمي لعلماء الأرض، وفيه قدمت بحوث عن تأثير ذوبان الجبال الجليدية العملاقة في القطب المتجمد الجنوبي.
ويرى العلماء المشاركون في المؤتمر أن لذوبان تلك الجبال الجليدية تأثيرات دراماتيكية، فهي تغير حتى طبيعة مخزونات غذاء الحيوانات البحرية ويأخذون على ذلك مثلا انتقال تلك الجبال الجليدية العملاقة المتكسرة نحو جزيرة جورجيا الجنوبية الموقع الذي تتلاشى فيه تلك الجبال. إذ تنجرف الصخور الجليدية الهائلة بعد انفصالها من القطب المتجمد الجنوبي لتستقر على الجرف القاري الضحل الذي يحيط الجزيرة التي يبلغ طولها 170 كيلومترا. وعندما تنهار هذه الصخور وتذوب تلقي بمليارات الأطنان من المياه العذبة في البيئة البحرية القريبة مما يغير طبيعة تلك البيئة.
ويقول عالم المحيطات في الجامعة المفتوحة، الدكتور مارك براندون في دراسة قدمها في المؤتمر إن الجبال الجليدية "هائلة فعلا إذ تبلغ كتلتها نحو 300 غيغا طن. وهي لا تنكسر إلى جزءين فحسب بل تتشظى إلى كتل عديدة أصغر، لكنها تظل ضخمة وتساهم في إلقاء كميات كبيرة من المياه العذبة إلى النظام البيئي المحلي".
وتضمنت دراسة الدكتور براندون وفريقه العلمي زرع مراسٍ خاصة في مئات الأمتار من مياه جزيرة جورجيا الجنوبية. وتحمل هذه المراسي مجسات استشعار لمراقبة الصفات الفيزيائية للماء، بما في ذلك درجة الحرارة والملوحة وسرعة التدفق. كما جرى قياس كميات الأحياء مثل بعض أنواع الطحالب. وقد وضعت المراسي في أفضل مكان لالتقاط ما يحدث عندما ظهر نوع أيه - 38 من الكتل الجليدية الضخمة عام 2004. وهذا نوع من الكتل الجليدية التي تضم أيضا كتل بي 10 أيه وأيه 22 بي، التي شوهدت في جورجيا الجنوبية، قادمة من القطب المتجمد الجنوبي. ويمتد الجرف القاري للجزيرة الى أكثر من 50 كيلومترا من الساحل ويبلغ معدل عمقه 200 متر، وعندما تصل الجبال الجليدية العملاقة الى الجزيرة، تستقر وتتآكل ببطء.
ويقول البحث العلمي إن للمياه العذبة تأثيرا قياسيا فهي تغير التيارات في الجرف القاري وكذلك كثافة ماء البحر. وفضلا عن ذلك تجعل ماء البحر أكثر برودة. ومن المحتمل أن تضيف كتل أيه 38 الجليدية 100 مليار طن من المياه العذبة إلى المنطقة المحيطة. ويقول البروفيسور أوجين ميرفي من مركز "مسح القطب الجنوبي" في بريطانيا، إن للكتل الضخمة تأثيرات بيولوجية مهمة. فالغبار وشظايا الصخور الجليدية الملتقطة في القطب الجنوبي تعمل كغذاء عندما تذوب في المحيط، مما يبث الحياة في أنواع الأشنات والعوالق والطحالب في عمق الشبكة الغذائية. لكن في جزيرة جورجيا الجنوبية، قد تكون هناك أحيانا عواقب سلبية خصوصا مع كتل أيه 38 الجليدية.
وأدت بعض البيانات التي جمعت من قبل الباحثين في المنطقة إلى أن التفكير في أن الجزء الأعظم من الجبال الجليدية قد تكون بمثابة الحاجز في وجه تدفق أسماك الكريل الشبيهة بالروبيان. وتتبع هذه الأسماك تيارات الكتل الجليدية وهي تشكل مصدرا غذائيا مهما للكثير من حيوانات الجزيرة كالبطاريق والفقمات والطيور، وبغيابها يتأثر تكاثر هذه الحيوانات سلبا، وفي الأوقات السيئة ستمتلئ شواطئ الجزيرة بفراخ الطيور وجراء الحيوانات الميتة كما يعتقد البروفيسور ميرفي. وأوضح ميرفي أن هذه الجبال عندما تستقر على الجرف القاري فإنها تقف في وجه تلك الأسماك ما يجعل الوقت الذي يحدث فيه ذلك "قاسيا بالنسبة للحياة البحرية والتركيب البيولوجي للبيئة" هناك.

[دائمة الذكر]

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جزاك الله كل خير

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